diff --git a/cql.js b/cql.js
index 37035d3..77e05bb 100644
--- a/cql.js
+++ b/cql.js
@@ -9,28 +9,51 @@ function indent(n, c) {
         s = s + c;
     return s;
 }
+
+var CQLError = function (message, query, pos, look, val, lval) {
+    this.name = "CQLError";
+    this.message = message;
+    // raw query string
+    this.query = query;
+    // parser (error) position in raw query
+    this.pos = pos;
+    // state: s | q | | ()/ | <>= | "'
+    this.look = look;
+    // value of token (may be (quoted) content, "index"; field or term)
+    this.val = val;
+    // lowercased this.val
+    this.lval = lval;
+    this.stack = (new Error()).stack;
+}
+CQLError.prototype = Object.create(Error.prototype);
+
 // CQLModifier
 var CQLModifier = function () {
+    // name of (relation) modifier
     this.name = null;
+    // relation of modifier value (e.g. equals, lower than, etc.)
     this.relation = null;
+    // optional value of modifier ("true" if empty)
     this.value = null;
+    this._pos = null;
+    this._range = null;
 }
 
 CQLModifier.prototype = {
     toString: function () {
-      return this.name + this.relation + this.value;
+        return this.name + this.relation + this.value;
     },
 
     toXCQL: function (n, c) {
-        var s = indent(n+1, c) + "<modifier>\n";
-        s = s + indent(n+2, c) + "<name>" + this.name + "</name>\n";
+        var s = indent(n + 1, c) + "<modifier>\n";
+        s = s + indent(n + 2, c) + "<name>" + this.name + "</name>\n";
         if (this.relation != null)
-            s = s + indent(n+2, c) 
+            s = s + indent(n + 2, c)
                 + "<relation>" + this.relation + "</relation>\n";
         if (this.value != null)
-            s = s + indent(n+2, c) 
-                + "<value>" + this.value +"</value>\n";
-        s = s + indent(n+1, c) + "</modifier>\n";
+            s = s + indent(n + 2, c)
+                + "<value>" + this.value + "</value>\n";
+        s = s + indent(n + 1, c) + "</modifier>\n";
         return s;
     },
 
@@ -38,170 +61,208 @@ CQLModifier.prototype = {
         //we ignore modifier relation symbol, for value-less modifiers
         //we assume 'true'
         var value = this.value.length > 0 ? this.value : "true";
-        var s = '"'+this.name+'": "'+value+'"';
+        var s = '"' + this.name + '": "' + value + '"';
+        if (this._pos !== null) s += ', "' + this.name + '@pos": ' + this._pos;
+        if (this._range !== null) s += ', "' + this.name + '@range": ' + JSON.stringify(this._range);
         return s;
     }
 }
 
 // CQLSearchClause
-var CQLSearchClause = function (field, fielduri, relation, relationuri, 
-                                modifiers, term, scf, scr) {
+var CQLSearchClause = function (field, fielduri, relation, relationuri,
+    modifiers, term, scf, scr) {
+    // index or name
     this.field = field;
+    // not supported yet? (always empty string)
     this.fielduri = fielduri;
+    // relation symbol or string
     this.relation = relation;
+    // not supported yet? (always empty string)
     this.relationuri = relationuri;
+    // list of modifiers
     this.modifiers = modifiers;
+    // value
     this.term = term;
     this.scf = scf || DEFAULT_SERVER_CHOICE_FIELD;
     this.scr = scr || DEFAULT_SERVER_CHOICE_RELATION;
+    // start position of search clause
+    this._pos = null;
+    // range (start + end position) of search clause (without surrounding braces or spaces)
+    this._range = null;
+    // start position of relation
+    this._relpos = null;
 }
 
 CQLSearchClause.prototype = {
     toString: function () {
-      var field = this.field;
-      var relation = this.relation;
-      if (field == this.scf && relation == this.scr) {
-        //avoid redundant field/relation
-        field = null;
-        relation = null;
-      }
-      return (field ? field + ' ' : '') + 
-        (relation ? relation : '') +
-        (this.modifiers.length > 0 ? '/' + this.modifiers.join('/') : '') +
-        (relation || this.modifiers.length ? ' ' : '') +
-        '"' + this.term + '"';
+        var field = this.field;
+        var relation = this.relation;
+        if (field == this.scf && relation == this.scr) {
+            //avoid redundant field/relation
+            field = null;
+            relation = null;
+        }
+        return (field ? field + ' ' : '') +
+            (relation ? relation : '') +
+            (this.modifiers.length > 0 ? '/' + this.modifiers.join('/') : '') +
+            (relation || this.modifiers.length ? ' ' : '') +
+            '"' + this.term + '"';
     },
 
     toXCQL: function (n, c) {
         var s = indent(n, c) + "<searchClause>\n";
-        if (this.fielduri.length > 0)
-        {
-            s = s + indent(n+1, c) + "<prefixes>\n" +
-                indent(n+2, c) + "<prefix>\n" +
-                indent(n+3, c) + "<identifier>" + this.fielduri +
+        if (this.fielduri.length > 0) {
+            s = s + indent(n + 1, c) + "<prefixes>\n" +
+                indent(n + 2, c) + "<prefix>\n" +
+                indent(n + 3, c) + "<identifier>" + this.fielduri +
                 "</identifier>\n" +
-                indent(n+2, c) + "</prefix>\n" +
-                indent(n+1, c) + "</prefixes>\n";
+                indent(n + 2, c) + "</prefix>\n" +
+                indent(n + 1, c) + "</prefixes>\n";
         }
-        s = s + indent(n+1, c) + "<index>" + this.field + "</index>\n";
-        s = s + indent(n+1, c) + "<relation>\n";
+        s = s + indent(n + 1, c) + "<index>" + this.field + "</index>\n";
+        s = s + indent(n + 1, c) + "<relation>\n";
         if (this.relationuri.length > 0) {
-            s = s + indent(n+2, c) +
+            s = s + indent(n + 2, c) +
                 "<identifier>" + this.relationuri + "</identifier>\n";
         }
-        s = s + indent(n+2, c) + "<value>" + this.relation + "</value>\n";
+        s = s + indent(n + 2, c) + "<value>" + this.relation + "</value>\n";
         if (this.modifiers.length > 0) {
-            s = s + indent(n+2, c) + "<modifiers>\n";
+            s = s + indent(n + 2, c) + "<modifiers>\n";
             for (var i = 0; i < this.modifiers.length; i++)
-                s = s + this.modifiers[i].toXCQL(n+2, c);
-            s = s + indent(n+2, c) + "</modifiers>\n";
+                s = s + this.modifiers[i].toXCQL(n + 2, c);
+            s = s + indent(n + 2, c) + "</modifiers>\n";
         }
-        s = s + indent(n+1, c) + "</relation>\n";
-        s = s + indent(n+1, c) + "<term>" + this.term + "</term>\n";
+        s = s + indent(n + 1, c) + "</relation>\n";
+        s = s + indent(n + 1, c) + "<term>" + this.term + "</term>\n";
         s = s + indent(n, c) + "</searchClause>\n";
         return s;
     },
 
     toFQ: function () {
-        var s = '{"term": "'+this.term+'"';
+        var s = '{"term": "' + this.term + '"';
         if (this.field.length > 0 && this.field != this.scf)
-          s+= ', "field": "'+this.field+'"';
+            s += ', "field": "' + this.field + '"';
         if (this.relation.length > 0 && this.relation != this.scr)
-          s+= ', "relation": "'+this._mapRelation(this.relation)+'"';
+            s += ', "relation": "' + this._mapRelation(this.relation) + '"';
         for (var i = 0; i < this.modifiers.length; i++) {
-          //since modifiers are mapped to keys, ignore the reserved ones
-          if (this.modifiers[i].name == "term"
-            ||this.modifiers[i].name == "field"
-            ||this.modifiers[i].name == "relation")
-            continue;
-          s += ', ' + this.modifiers[i].toFQ();
+            //since modifiers are mapped to keys, ignore the reserved ones
+            if (this.modifiers[i].name == "term"
+                || this.modifiers[i].name == "field"
+                || this.modifiers[i].name == "relation")
+                continue;
+            s += ', ' + this.modifiers[i].toFQ();
         }
+        if (this._pos !== null) s += ', "@pos": ' + this._pos;
+        if (this._range !== null) s += ', "@range": ' + JSON.stringify(this._range);
+        if (this._relpos !== null) s += ', "relation@pos": ' + this._relpos;
         s += '}';
         return s;
     },
 
     _mapRelation: function (rel) {
-      switch(rel) {
-        case "<" : return "lt";
-        case ">" : return "gt";
-        case "=" : return "eq";
-        case "<>" : return "ne";
-        case ">=" : return "ge";
-        case "<=" : return "le";
-        default: return rel;
-      }
+        switch (rel) {
+            case "<": return "lt";
+            case ">": return "gt";
+            case "=": return "eq";
+            case "<>": return "ne";
+            case ">=": return "ge";
+            case "<=": return "le";
+            default: return rel;
+        }
     },
 
     _remapRelation: function (rel) {
-      switch(rel) {
-        case "lt" : return "<";
-        case "gt" : return ">";
-        case "eq" : return "=";
-        case "ne" : return "<>";
-        case "ge" : return ">=";
-        case "le" : return "<=";
-        default: return rel;
-      }
+        switch (rel) {
+            case "lt": return "<";
+            case "gt": return ">";
+            case "eq": return "=";
+            case "ne": return "<>";
+            case "ge": return ">=";
+            case "le": return "<=";
+            default: return rel;
+        }
     }
-
 }
+
 // CQLBoolean
-var CQLBoolean = function() {
+var CQLBoolean = function () {
+    // operator: and | or | not | prox
     this.op = null;
+    // list of modifiers
     this.modifiers = null;
+    // left search clause
     this.left = null;
+    // right search clause
     this.right = null;
+    this._pos = null;
+    this._range = null;
 }
 
 CQLBoolean.prototype = {
     toString: function () {
-      return (this.left.op ? '(' + this.left + ')' : this.left) + ' ' + 
-        this.op.toUpperCase() +
-        (this.modifiers.length > 0 ? '/' + this.modifiers.join('/') : '') + 
-        ' ' + (this.right.op ? '(' + this.right + ')' : this.right);;
+        return (this.left.op ? '(' + this.left + ')' : this.left) + ' ' +
+            this.op.toUpperCase() +
+            (this.modifiers.length > 0 ? '/' + this.modifiers.join('/') : '') +
+            ' ' + (this.right.op ? '(' + this.right + ')' : this.right);;
     },
+
     toXCQL: function (n, c) {
         var s = indent(n, c) + "<triple>\n";
-        s = s + indent(n+1, c) + "<boolean>\n" +
-            indent(n+2, c) + "<value>" + this.op + "</value>\n";
+        s = s + indent(n + 1, c) + "<boolean>\n" +
+            indent(n + 2, c) + "<value>" + this.op + "</value>\n";
         if (this.modifiers.length > 0) {
-            s = s + indent(n+2, c) + "<modifiers>\n";
+            s = s + indent(n + 2, c) + "<modifiers>\n";
             for (var i = 0; i < this.modifiers.length; i++)
-                s = s + this.modifiers[i].toXCQL(n+2, c);
-            s = s + indent(n+2, c) + "</modifiers>\n";
+                s = s + this.modifiers[i].toXCQL(n + 2, c);
+            s = s + indent(n + 2, c) + "</modifiers>\n";
         }
-        s = s + indent(n+1, c) + "</boolean>\n";
-        s = s + indent(n+1, c) + "<leftOperand>\n" +
-            this.left.toXCQL(n+2, c) + indent(n+1, c) + "</leftOperand>\n";
+        s = s + indent(n + 1, c) + "</boolean>\n";
+        s = s + indent(n + 1, c) + "<leftOperand>\n" +
+            this.left.toXCQL(n + 2, c) + indent(n + 1, c) + "</leftOperand>\n";
 
-        s = s + indent(n+1, c) + "<rightOperand>\n" +
-            this.right.toXCQL(n+2, c) + indent(n+1, c) + "</rightOperand>\n";
+        s = s + indent(n + 1, c) + "<rightOperand>\n" +
+            this.right.toXCQL(n + 2, c) + indent(n + 1, c) + "</rightOperand>\n";
         s = s + indent(n, c) + "</triple>\n";
         return s;
     },
 
     toFQ: function (n, c, nl) {
-      var s = '{"op": "'+this.op+'"';
-      //proximity modifiers
-      for (var i = 0; i < this.modifiers.length; i++)
-        s += ', ' + this.modifiers[i].toFQ();
-      s += ','+nl+indent(n, c)+' "s1": '+this.left.toFQ(n+1, c, nl);
-      s += ','+nl+indent(n, c)+' "s2": '+this.right.toFQ(n+1, c, nl);
-      var fill = n && c ? ' ' : '';
-      s += nl+indent(n-1, c)+fill+'}';
-      return s;
+        var s = '{"op": "' + this.op + '"';
+        //proximity modifiers
+        for (var i = 0; i < this.modifiers.length; i++)
+            s += ', ' + this.modifiers[i].toFQ();
+        s += ',' + nl + indent(n, c) + ' "s1": ' + this.left.toFQ(n + 1, c, nl);
+        s += ',' + nl + indent(n, c) + ' "s2": ' + this.right.toFQ(n + 1, c, nl);
+        if (this._pos !== null) s += ',' + nl + indent(n, c) + ' "@pos": ' + this._pos;
+        if (this._range !== null) s += ',' + nl + indent(n, c) + ' "@range": ' + JSON.stringify(this._range);
+        var fill = n && c ? ' ' : '';
+        s += nl + indent(n - 1, c) + fill + '}';
+        return s;
     }
 
 }
+
 // CQLParser
 var CQLParser = function () {
+    // index/position in raw query
     this.qi = null;
+    // raw query string length (char count)
     this.ql = null;
+    // raw query string
     this.qs = null;
+    // parser state: s | q | ()/ | <>= | "'
+    // if empty string then error
     this.look = null;
+    // lowercased this.val
     this.lval = null;
+    // value of token (may be (quoted) content, "index"; field or term)
     this.val = null;
+    // (internal) start position of expression
+    this._exprStart = null;
+    // (internal) start position of relation (in expression)
+    this._exprRelStart = null;
     this.prefixes = new Object();
+    // parsed query
     this.tree = null;
     this.scf = null;
     this.scr = null;
@@ -210,78 +271,83 @@ var CQLParser = function () {
 CQLParser.prototype = {
     parse: function (query, scf, scr) {
         if (!query)
-            throw new Error("The query to be parsed cannot be empty");
-        this.scf = typeof scf != 'string' 
-          ? DEFAULT_SERVER_CHOICE_FIELD : scf;
-        this.scr = typeof scr != 'string' 
-          ? DEFAULT_SERVER_CHOICE_RELATION : scr; 
+            throw new CQLError("The query to be parsed cannot be empty", this.qs);
+        this.scf = typeof scf != 'string'
+            ? DEFAULT_SERVER_CHOICE_FIELD : scf;
+        this.scr = typeof scr != 'string'
+            ? DEFAULT_SERVER_CHOICE_RELATION : scr;
         this.qs = query;
         this.ql = this.qs.length;
         this.qi = 0;
-        this._move(); 
+        this.lval = this.val = this._exprStart = null;
+        this._move();
         this.tree = this._parseQuery(this.scf, this.scr, new Array());
         if (this.look != "")
-            throw new Error("EOF expected");
+            throw new CQLError("EOF expected", this.qs, this.qi, this.look, this.val, this.lval);
     },
     parseFromFQ: function (query, scf, scr) {
-       if (!query)
-          throw new Error("The query to be parsed cannot be empty");
-       if (typeof query == 'string')
-         query = JSON.parse(query);
-       this.scf = typeof scf != 'string' 
-         ? DEFAULT_SERVER_CHOICE_FIELD : scf;
-       this.scr = typeof scr != 'string' 
-         ? DEFAULT_SERVER_CHOICE_RELATION : scr;
-       this.tree = this._parseFromFQ(query);
+        if (!query)
+            throw new CQLError("The query to be parsed cannot be empty", this.qs);
+        if (typeof query == 'string')
+            query = JSON.parse(query);
+        this.scf = typeof scf != 'string'
+            ? DEFAULT_SERVER_CHOICE_FIELD : scf;
+        this.scr = typeof scr != 'string'
+            ? DEFAULT_SERVER_CHOICE_RELATION : scr;
+        this.tree = this._parseFromFQ(query);
     },
     _parseFromFQ: function (fq) {
         //op-node
-        if (fq.hasOwnProperty('op') 
+        if (fq.hasOwnProperty('op')
             && fq.hasOwnProperty('s1')
             && fq.hasOwnProperty('s2')) {
-          var node = new CQLBoolean();
-          node.op = fq.op;
-          node.left = this._parseFromFQ(fq.s1);
-          node.right = this._parseFromFQ(fq.s2);
-          //include all other members as modifiers
-          node.modifiers = [];
-          for (var key in fq) {
-            if (key == 'op' || key == 's1' || key == 's2')
-              continue;
-            var mod = new CQLModifier();
-            mod.name = key;
-            mod.relation = '=';
-            mod.value = fq[key];
-            node.modifiers.push(mod);
-          }
-          return node;
+            var node = new CQLBoolean();
+            node.op = fq.op;
+            node.left = this._parseFromFQ(fq.s1);
+            node.right = this._parseFromFQ(fq.s2);
+            //include all other members as modifiers
+            node.modifiers = [];
+            for (var key in fq) {
+                if (key == 'op' || key == 's1' || key == 's2')
+                    continue;
+                if (key.endsWith("@pos") || key.endsWith("@range"))
+                    continue;
+                var mod = new CQLModifier();
+                mod.name = key;
+                mod.relation = '=';
+                mod.value = fq[key];
+                node.modifiers.push(mod);
+            }
+            return node;
         }
         //search-clause node
         if (fq.hasOwnProperty('term')) {
-          var node = new CQLSearchClause();
-          node.term = fq.term;
-          node.scf = this.scf;
-          node.scr = this.scr;
-          node.field = fq.hasOwnProperty('field') 
-            ? fq.field : this.scf;
-          node.relation = fq.hasOwnProperty('relation')
-            ? node._remapRelation(fq.relation) : this.scr;
-          //include all other members as modifiers
-          node.relationuri = '';
-          node.fielduri = '';
-          node.modifiers = [];
-          for (var key in fq) {
-            if (key == 'term' || key == 'field' || key == 'relation')
-              continue;
-            var mod = new CQLModifier();
-            mod.name = key;
-            mod.relation = '=';
-            mod.value = fq[key];
-            node.modifiers.push(mod);
-          }
-          return node;
+            var node = new CQLSearchClause();
+            node.term = fq.term;
+            node.scf = this.scf;
+            node.scr = this.scr;
+            node.field = fq.hasOwnProperty('field')
+                ? fq.field : this.scf;
+            node.relation = fq.hasOwnProperty('relation')
+                ? node._remapRelation(fq.relation) : this.scr;
+            //include all other members as modifiers
+            node.relationuri = '';
+            node.fielduri = '';
+            node.modifiers = [];
+            for (var key in fq) {
+                if (key == 'term' || key == 'field' || key == 'relation')
+                    continue;
+                if (key.endsWith("@pos") || key.endsWith("@range"))
+                    continue;
+                var mod = new CQLModifier();
+                mod.name = key;
+                mod.relation = '=';
+                mod.value = fq[key];
+                node.modifiers.push(mod);
+            }
+            return node;
         }
-        throw new Error('Unknow node type; '+JSON.stringify(fq));
+        throw new CQLError('Unknow node type; ' + JSON.stringify(fq), this.qs, this.qi, this.look, this.val, this.lval);
     },
     toXCQL: function (c) {
         c = typeof c == "undefined" ? ' ' : c;
@@ -295,40 +361,51 @@ CQLParser.prototype = {
     toString: function () {
         return this.tree.toString();
     },
-    _parseQuery: function(field, relation, modifiers) {
+    _parseQuery: function (field, relation, modifiers) {
         var left = this._parseSearchClause(field, relation, modifiers);
         while (this.look == "s" && (
-                    this.lval == "and" ||
-                    this.lval == "or" ||
-                    this.lval == "not" ||
-                    this.lval == "prox")) {
+            this.lval == "and" ||
+            this.lval == "or" ||
+            this.lval == "not" ||
+            this.lval == "prox")) {
             var b = new CQLBoolean();
+            b._pos = this.qi - this.lval.length + 1;
             b.op = this.lval;
             this._move();
             b.modifiers = this._parseModifiers();
+            b._range = [b._pos, Math.max(b._pos + b.op.length - 1, Math.max.apply(null, b.modifiers.map(x => ("_range" in x) ? x._range[1]: 0)))];
             b.left = left;
             b.right = this._parseSearchClause(field, relation, modifiers);
             left = b;
         }
         return left;
     },
-    _parseModifiers: function() {
+    _parseModifiers: function () {
         var ar = new Array();
+        let _safeExprStart = this._exprStart;
+        let _safeExprRelStart = this._exprRelStart;
         while (this.look == "/") {
+            let _mstart = this.qi;
             this._move();
             if (this.look != "s" && this.look != "q")
-                throw new Error("Invalid modifier.")
-            
+                throw new CQLError("Invalid modifier.", this.qs, this.qi, this.look, this.val, this.lval)
+
             var name = this.lval;
+            let _mpos = this.qi - this.lval.length + 1;
+            let _mend = this.qi;
             this._move();
-            if (this.look.length > 0 
+            if (this.look.length > 0
                 && this._strchr("<>=", this.look.charAt(0))) {
                 var rel = this.look;
                 this._move();
+                _mend = this.qi;
                 if (this.look != "s" && this.look != "q")
-                    throw new Error("Invalid relation within the modifier.");
-                
+                    throw new CQLError("Invalid relation within the modifier.", this.qs, this.qi, this.look, this.val, this.lval);
+
                 var m = new CQLModifier();
+                m._pos = _mpos;
+                m._range = [_mstart, _mend];
+                this._exprStart = null;
                 m.name = name;
                 m.relation = rel;
                 m.value = this.val;
@@ -336,54 +413,64 @@ CQLParser.prototype = {
                 this._move();
             } else {
                 var m = new CQLModifier();
+                m._pos = _mpos;
+                m._range = [_mstart, _mend];
+                this._exprStart = null;
                 m.name = name;
                 m.relation = "";
                 m.value = "";
                 ar.push(m);
             }
         }
+        this._exprStart = _safeExprStart;
+        this._exprRelStart = _safeExprRelStart;
         return ar;
     },
-    _parseSearchClause: function(field, relation, modifiers) {
+    _parseSearchClause: function (field, relation, modifiers) {
         if (this.look == "(") {
+            let _qi = this.qi;
             this._move();
             var b = this._parseQuery(field, relation, modifiers);
             if (this.look == ")")
                 this._move();
             else
-                throw new Error("Missing closing parenthesis.");
+                throw new CQLError("Missing closing parenthesis.", this.qs, _qi, this.look, this.val, this.lval);
 
             return b;
         } else if (this.look == "s" || this.look == "q") {
             var first = this.val;   // dont know if field or term yet
+            if (this._exprStart === null)
+                this._exprStart = this.qi - this.val.length + 1;
+            let _tend = this.qi;
             this._move();
             if (this.look == "q" ||
-                    (this.look == "s" &&
-                     this.lval != "and" &&
-                     this.lval != "or" &&
-                     this.lval != "not" &&
-                     this.lval != "prox")) {
+                (this.look == "s" &&
+                    this.lval != "and" &&
+                    this.lval != "or" &&
+                    this.lval != "not" &&
+                    this.lval != "prox")) {
                 var rel = this.val;    // string relation
+                this._exprRelStart = this.qi - this.val.length + 1;
                 this._move();
                 return this._parseSearchClause(first, rel,
-                                               this._parseModifiers());
-            } else if (this.look.length > 0 
-                       && this._strchr("<>=", this.look.charAt(0))) {
+                    this._parseModifiers());
+            } else if (this.look.length > 0
+                && this._strchr("<>=", this.look.charAt(0))) {
                 var rel = this.look;   // other relation <, = ,etc
                 this._move();
-                return this._parseSearchClause(first, rel, 
-                                               this._parseModifiers());
+                return this._parseSearchClause(first, rel,
+                    this._parseModifiers());
             } else {
                 // it's a search term
                 var pos = field.indexOf('.');
                 var pre = "";
                 if (pos != -1)
                     pre = field.substring(0, pos);
-                
+
                 var uri = this._lookupPrefix(pre);
                 if (uri.length > 0)
-                    field = field.substring(pos+1);
-                
+                    field = field.substring(pos + 1);
+
                 pos = relation.indexOf('.');
                 if (pos == -1)
                     pre = "cql";
@@ -392,32 +479,34 @@ CQLParser.prototype = {
 
                 var reluri = this._lookupPrefix(pre);
                 if (reluri.Length > 0)
-                    relation = relation.Substring(pos+1);
+                    relation = relation.Substring(pos + 1);
 
                 var sc = new CQLSearchClause(field,
-                        uri,
-                        relation,
-                        reluri,
-                        modifiers,
-                        first,
-                        this.scf,
-                        this.scr);
+                    uri,
+                    relation,
+                    reluri,
+                    modifiers,
+                    first,
+                    this.scf,
+                    this.scr);
+                sc._range = [this._exprStart, _tend];
+                sc._relpos = this._exprRelStart;
+                this._exprStart = this._exprRelStart = null;
                 return sc;
             }
-        // prefixes
+            // prefixes
         } else if (this.look == ">") {
             this._move();
             if (this.look != "s" && this.look != "q")
-                throw new Error("Expecting string or a quoted expression.");
-            
+                throw new CQLError("Expecting string or a quoted expression.", this.qs, this.qi, this.look, this.val, this.lval);
+
             var first = this.lval;
             this._move();
-            if (this.look == "=")
-            {
+            if (this.look == "=") {
                 this._move();
                 if (this.look != "s" && this.look != "q")
-                    throw new Error("Expecting string or a quoted expression.");
-                
+                    throw new CQLError("Expecting string or a quoted expression.", this.qs, this.qi, this.look, this.val, this.lval);
+
                 this._addPrefix(first, this.lval);
                 this._move();
                 return this._parseQuery(field, relation, modifiers);
@@ -426,14 +515,14 @@ CQLParser.prototype = {
                 return this._parseQuery(field, relation, modifiers);
             }
         } else {
-            throw new Error("Invalid search clause.");
+            throw new CQLError("Invalid search clause.", this.qs, this.qi, this.look, this.val, this.lval);
         }
 
     },
     _move: function () {
         //eat whitespace
-        while (this.qi < this.ql 
-               && this._strchr(" \t\r\n", this.qs.charAt(this.qi)))
+        while (this.qi < this.ql
+            && this._strchr(" \t\r\n", this.qs.charAt(this.qi)))
             this.qi++;
         //eof
         if (this.qi == this.ql) {
@@ -446,18 +535,21 @@ CQLParser.prototype = {
         if (this._strchr("()/", c)) {
             this.look = c;
             this.qi++;
-        //comparitor
+            //comparitor
         } else if (this._strchr("<>=", c)) {
             this.look = c;
             this.qi++;
+            this._exprRelStart = this.qi;
             //comparitors can repeat, could be if
-            while (this.qi < this.ql 
-                   && this._strchr("<>=", this.qs.charAt(this.qi))) {
+            while (this.qi < this.ql
+                && this._strchr("<>=", this.qs.charAt(this.qi))) {
                 this.look = this.look + this.qs.charAt(this.qi);
                 this.qi++;
             }
-        //quoted string
+            //quoted string
         } else if (this._strchr("\"'", c)) {
+            if (this._exprStart === null)
+                this._exprStart = this.qi + 1;
             this.look = "q";
             //remember quote char
             var mark = c;
@@ -465,26 +557,26 @@ CQLParser.prototype = {
             this.val = "";
             var escaped = false;
             while (this.qi < this.ql) {
-              if (!escaped && this.qs.charAt(this.qi) == mark)
-                break;
-              if (!escaped && this.qs.charAt(this.qi) == '\\')
-                escaped = true;
-              else
-                escaped = false;
-              this.val += this.qs.charAt(this.qi);
-              this.qi++;
+                if (!escaped && this.qs.charAt(this.qi) == mark)
+                    break;
+                if (!escaped && this.qs.charAt(this.qi) == '\\')
+                    escaped = true;
+                else
+                    escaped = false;
+                this.val += this.qs.charAt(this.qi);
+                this.qi++;
             }
             this.lval = this.val.toLowerCase();
             if (this.qi < this.ql)
                 this.qi++;
             else //unterminated
-              this.look = ""; //notify error
-        //unquoted string
+                this.look = ""; //notify error
+            //unquoted string
         } else {
             this.look = "s";
             this.val = "";
-            while (this.qi < this.ql 
-                   && !this._strchr("()/<>= \t\r\n", this.qs.charAt(this.qi))) {
+            while (this.qi < this.ql
+                && !this._strchr("()/<>= \t\r\n", this.qs.charAt(this.qi))) {
                 this.val = this.val + this.qs.charAt(this.qi);
                 this.qi++;
             }
@@ -494,11 +586,14 @@ CQLParser.prototype = {
     _strchr: function (s, ch) {
         return s.indexOf(ch) >= 0
     },
-    _lookupPrefix: function(name) {
+    _lookupPrefix: function (name) {
         return this.prefixes[name] ? this.prefixes[name] : "";
     },
-    _addPrefix: function(name, value) {
+    _addPrefix: function (name, value) {
         //overwrite existing items
         this.prefixes[name] = value;
     }
 }
+
+// for module import systems uncomment the following line
+// module.exports = CQLParser;
diff --git a/index.html b/index.html
index 50aaef1..34eec2d 100644
--- a/index.html
+++ b/index.html
@@ -4,6 +4,48 @@
     <script type="text/javascript">
         var cp = new CQLParser();
         
+        function buildVis(query) {
+          var cp = new CQLParser();
+          cp.parse(query);
+          var fq = JSON.parse(cp.toFQ());
+
+          function recMods (obj, s, lvl) {
+            if (lvl === undefined) lvl = 0;
+            if ("term" in obj) {
+              if ("@range" in obj) {
+                let range = obj["@range"];
+                s += '\n' + ("".padStart(range[0] - 1) + "t".padEnd(range[1] - range[0] + 1, "-")).padEnd(query.length + 1 + lvl) + "[term:" + range + "]"
+              }
+              if ("relation@pos" in obj) {
+                let relpos = obj["relation@pos"];
+                let relmodend = Math.max(...Object.entries(obj).filter(x => x[0].endsWith("@range") && x[0].length > 6).map(([_, range]) => range[1]));
+                s += '\n' + ("".padStart(relpos - 1) + "r".padEnd(relmodend - relpos + 1, "-")).padEnd(query.length + 1 + lvl + 1) + "[term:rel:" + obj["relation"] + ":" + relpos + "]"
+              }
+              Object.entries(obj).filter(x => x[0].endsWith("@range") && x[0].length > 6).map(([_, range]) => {
+                s += '\n' + ("".padStart(range[0] - 1) + "m".padEnd(range[1] - range[0] + 1, "-")).padEnd(query.length + 1 + lvl + 2) + "[term:mod:" + range + "]"
+              })
+            } else if ("op" in obj) {
+              if ("@range" in obj) {
+                let range = obj["@range"];
+                s += '\n' + ("".padStart(range[0] - 1) + "o".padEnd(range[1] - range[0] + 1, "-")).padEnd(query.length + 1 + lvl) + "[op:" + range + "]"
+              } else if ("@pos" in obj) {
+                let range = [obj["@pos"], obj["@pos"] + obj.op.length - 1]
+                s += '\n' + ("o".padEnd(obj.op.length, "-").padStart(obj["@pos"] + obj.op.length - 1)).padEnd(query.length + 1 + lvl) + "[op:" + range + "]"
+              }
+              Object.entries(obj).filter(x => x[0].endsWith("@range")  && x[0].length > 6).map(([_, range]) => {
+                s += '\n' + ("".padStart(range[0] - 1) + "m".padEnd(range[1] - range[0] + 1, "-")).padEnd(query.length + 1 + lvl + 1) + "[op:mod:" + range + "]"
+              })
+              s = recMods(obj.s1, s, lvl+1);
+              s = recMods(obj.s2, s, lvl+1);
+            }
+            return s;
+          }
+
+          var vis = query;
+          vis = recMods(fq, vis);
+          return vis;
+        }
+
         function parseInput(query) {
             try {
                 cp.parse(query);
@@ -14,13 +56,43 @@
                 document.getElementById("output3").value = fq;
                 cp.parseFromFQ(fq);
                 document.getElementById("output4").value = cp.toString();
+                // visualize positions/ranges in original query
+                document.getElementById("output5").value = buildVis(query);
             } catch (e) {
-                document.getElementById("output").value = e.message;
-                document.getElementById("output2").value = e.message;
+                var msg = e.toString();
+                msg += "\n\n\tquery: " + e.query + "\n\t       " + "^".padStart(e.pos) + " (pos: " + e.pos + ")";
+                msg += "\n\n\tcontext: " + JSON.stringify({"look": e.look, "val": e.val, "lval": e.lval});
+                document.getElementById("output").value = msg;
+                document.getElementById("output2").value = "";
+                document.getElementById("output3").value = "";
+                document.getElementById("output4").value = "";
+                document.getElementById("output5").value = "";
                 throw e;
             }
         }
     </script>
+    <style type="text/css">
+      .output {
+        position: relative;
+        width: 49%;
+        display: inline-block;
+      }
+      .output > textarea {
+        width: 100%;
+
+      }
+      .output > .label {
+        position: absolute;
+        top: 1ex;
+        right: 1ex;
+        padding: 3px;
+        margin: 0px;
+        color: #0000005e;
+        border: 1px solid #0000005e;
+        border-radius: 3px;
+        pointer-events: none;
+      }
+    </style>
     <title>CQL parser</title>
   </head>
   <body>
@@ -28,15 +100,27 @@
       <form name="test" onsubmit="parseInput(this.q.value); return false;">
         Query: 
         <input type="text" name="q" size="160" onkeyup="parseInput(this.value); return false;"></input>
-        <br>
-        <br>
-        XCQL:
-        <br>
       </form>
-      <textarea id="output" rows="80" cols="80"></textarea>
-      <textarea id="output2" rows="80" cols="80"></textarea>
-      <textarea id="output3" rows="80" cols="80"></textarea>
-      <textarea id="output4" rows="80" cols="80"></textarea>
+      <div class="output">
+        <div class="label">XCQL</div>
+        <textarea id="output" rows="10" cols="80" readonly></textarea>
+      </div>
+      <div class="output">
+        <div class="label">String</div>
+        <textarea id="output2" rows="10" cols="80" readonly></textarea>
+      </div>
+      <div class="output">
+        <div class="label">FQ</div>
+        <textarea id="output3" rows="10" cols="80" readonly></textarea>
+      </div>
+      <div class="output">
+        <div class="label">String (FQ-parsed)</div>
+        <textarea id="output4" rows="10" cols="80" readonly></textarea>
+      </div>
+      <div class="output">
+        <div class="label">Annotated<br/>(original input)</div>
+        <textarea id="output5" rows="8" cols="80" readonly></textarea>
+      </div>
     </div>
   </body>
 </html>