fixes source-academy#805 (source-academy#1012)

xml/chapter3/section3/subsection3.xml

@@ -826,6 +826,102 @@ display(get(list("b")));
     (e.g., numerically or alphabetically).  (Compare
     exercise<SPACE/><REF NAME="ex:set-lookup-binary-tree"/> of chapter<SPACE/><REF NAME="chap:data"></REF>.)
     <LABEL NAME="ex:3_26"/>
+    <SOLUTION>
+      <SNIPPET>
+	<NAME>ex_3_26_solution</NAME>
+	<EXAMPLE>ex_3_26_solution_example</EXAMPLE>
+	<JAVASCRIPT>
+// provided by GitHub user devinryu
+
+function entry(tree) { return head(tree); }
+function left_branch(tree) { return head(tail(tree)); }
+function right_branch(tree) { return head(tail(tail(tree))); }
+function make_tree(entry, left, right) { 
+    return list(entry, left, right);
+}
+
+// kv is list(key, value)
+function adjoin_set(kv, set) {
+    return is_null(set)
+           ? make_tree(kv, null, null)
+           : head(kv) === head(entry(set))
+           ? set
+           : head(kv) &lt; head(entry(set))
+           ? make_tree(entry(set),
+                       adjoin_set(kv, left_branch(set)),
+                       right_branch(set))
+           : make_tree(entry(set),
+                       left_branch(set),
+                       adjoin_set(kv, right_branch(set)));
+}
+
+function make_table() {
+    let local_table = null;
+    function lookup(given_key, tree_of_records) {
+        if (is_null(tree_of_records)) {
+            return null;
+        } else {
+            const this_entry = entry(tree_of_records);
+            const this_key = head(this_entry);
+            return given_key === this_key 
+                   ? this_entry
+                   : given_key &lt; this_key
+                   ? lookup(given_key, 
+                            left_branch(tree_of_records))
+                   : lookup(given_key, 
+                            right_branch(tree_of_records));
+        }
+    }
+    function insert(k, v) {
+        let record = lookup(k, local_table);
+        if(is_null(record)) {
+            local_table = adjoin_set(list(k, v), local_table);
+        } else {
+            // do nothing
+        }
+    }
+    function get(k) {
+        return head(tail(lookup(k, local_table)));
+    }
+    function print() {
+        return display(local_table);
+    }
+    function dispatch(m) {
+        return m === "lookup"
+        ? get
+        : m === "insert"
+        ? insert
+        : m  === "print"
+        ? print
+        : error(m, "error");
+    }
+    return dispatch;
+}
+	</JAVASCRIPT>
+      </SNIPPET>
+      <SNIPPET HIDE="yes">
+	<NAME>ex_3_26_solution_example</NAME>
+	<JAVASCRIPT>
+const t = make_table();
+const get = t("lookup");
+const put = t("insert");
+const print = t("print");
+
+// The test results
+
+put(3, "d");
+put(1, "a");
+put(2, "b");
+put(2, "c");
+put(4, "e");
+put(5, "f");
+
+print();
+
+display(get(2)); // displays: "b"
+	</JAVASCRIPT>
+      </SNIPPET>
+    </SOLUTION>
   </EXERCISE>
 
   <EXERCISE>