fixes #825 (#1020)

xml/chapter3/section3/subsection1.xml

@@ -1592,6 +1592,50 @@ display(count_pairs(cycle));
     would go into an infinite loop. Exercise<SPACE/><REF NAME="ex:make-cycle"/>
     constructed such lists.
     <LABEL NAME="ex:find-cycle"/>
+   <SOLUTION>
+      <SNIPPET>
+	<EXAMPLE>exercise_3_18_solution_example</EXAMPLE>
+	<JAVASCRIPT>
+// solution provided by GitHub user clean99
+
+function contains_cycle(x) {
+    let counted_pairs = null;
+    function is_counted_pair(counted_pairs, x) {
+        return is_null(counted_pairs)
+               ? false
+               : head(counted_pairs) === x
+               ? true
+               : is_counted_pair(tail(counted_pairs), x);
+    }
+    function detect_cycle(x) {
+        if (is_null(x)) {
+            return false;
+        } else if (is_counted_pair(counted_pairs, x)) {
+            return true;
+        } else {
+            counted_pairs = pair(x, counted_pairs);
+            return detect_cycle(tail(x));
+        }
+    }
+    return detect_cycle(x);
+}
+	</JAVASCRIPT>
+      </SNIPPET>
+      <SNIPPET HIDE="yes">
+         <NAME>exercise_3_18_solution_example</NAME>
+         <JAVASCRIPT>
+const three_list = list("a", "b", "c");
+const cycle = list("g", "h", "i");
+set_tail(tail(tail(cycle)), cycle);
+
+// displays false
+display(contains_cycle(three_list));
+
+// displays true
+display(contains_cycle(cycle));
+	 </JAVASCRIPT>
+      </SNIPPET>
+    </SOLUTION>
   </EXERCISE>
 
   <EXERCISE>