fixes #807 (#1013)

* fixes #807

* no eval here

Author: martin-henz

xml/chapter2/section5/subsection3.xml

@@ -1025,6 +1025,37 @@ head(tail(tail(tail(mul(p1, p2)))));
     subtraction of polynomials.
     (Hint: You may find it helpful to define a generic negation operation.)
     <LABEL NAME="ex:sub-poly"/>
+    <SOLUTION>
+      <SNIPPET EVAL="no">
+	<NAME>ex_2_88_solution</NAME>
+	<JAVASCRIPT>
+function sub_terms(L1, L2) {
+    if (is_empty_termlist(L1)) {
+        return L2;
+    } else if (is_empty_termlist(L2)) {
+        return L1;
+    } else {
+        const t1 = first_term(L1);
+        const t2 = first_term(L2);
+        return order(t1) &gt; order(t2)
+               ? adjoin_term(t1, sub_terms(rest_terms(L1), L2))
+               : order(t1) &lt; order(t2)
+               ? adjoin_term(t2, sub_terms(L1, rest_terms(L2)))
+               : adjoin_term(make_term(order(t1), 
+                                       sub(coeff(t1), coeff(t2))),
+                             sub_terms(rest_terms(L1),
+                                       rest_terms(L2)));
+    }
+}
+function sub_poly(p1, p2) {
+    return is_same_variable(variable(p1), variable(p2))
+           ? make_poly(variable(p1),
+                       sub_terms(term_list(p1), term_list(p2)))
+           : error(list(p1, p2), "polys not in same var -- sub_poly");
+}
+	</JAVASCRIPT>
+      </SNIPPET>
+    </SOLUTION>
   </EXERCISE>
 
   <EXERCISE>