fixes #824 (#1019)

Author: martin-henz

xml/chapter3/section3/subsection1.xml

@@ -1522,6 +1522,56 @@ display(count_pairs(cycle));
     distinct pairs in any structure.  (Hint: Traverse the structure, maintaining
     an auxiliary data structure that is used to keep track of which pairs have
     already been counted.)
+    <SOLUTION>
+      <SNIPPET>
+	<EXAMPLE>exercise_3_17_solution_example</EXAMPLE>
+	<JAVASCRIPT>
+// solution provided by GitHub user clean99
+	  
+function count_pairs(x) {
+    let counted_pairs = null;
+    function is_counted_pair(current_counted_pairs, x) {
+        return is_null(current_counted_pairs)
+               ? false
+               : head(current_counted_pairs) === x
+               ? true
+               : is_counted_pair(tail(current_counted_pairs), x);
+    }
+    function count(x) {
+        if(! is_pair(x) || is_counted_pair(counted_pairs, x)) {
+            return 0;
+        } else {
+            counted_pairs = pair(x, counted_pairs);
+            return count(head(x)) +
+                   count(tail(x)) +
+                   1;
+        }
+    }
+    return count(x);
+}
+	</JAVASCRIPT>
+      </SNIPPET>
+      <SNIPPET HIDE="yes">
+         <NAME>exercise_3_17_solution_example</NAME>
+         <JAVASCRIPT>
+const three_list = list("a", "b", "c");
+const one = pair("d", "e");
+const two = pair(one, one);
+const four_list = pair(two, "f");
+const seven_list = pair(two, two);
+const cycle = list("g", "h", "i");
+set_tail(tail(tail(cycle)), cycle);
+
+// return 3; return 3; return 3;
+display(count_pairs(three_list));
+display(count_pairs(four_list));
+display(count_pairs(seven_list));
+
+// return 3
+display(count_pairs(cycle));
+	 </JAVASCRIPT>
+      </SNIPPET>
+    </SOLUTION>
   </EXERCISE>
 
   <EXERCISE>