fixes #821 (#1016)

Author: martin-henz

xml/chapter2/section5/subsection1.xml

@@ -932,7 +932,54 @@ function contents(datum) {
     arithmetic package.  This operation should work for ordinary numbers,
     rational numbers, and complex numbers.
     <LABEL NAME="ex:equ?"/>
-  </EXERCISE>
+    <SOLUTION>
+      <SNIPPET EVAL="no">
+	<JAVASCRIPT>
+// provided by GitHub user clean99
+
+function is_equal(x, y) {
+    return apply_generic("is_equal", list(x, y));
+}
+
+function install_javascript_number_package() {
+    // ...
+
+    put("is_equal", list("javascript_number", "javascript_number"),
+           (x, y) => x === y);
+
+    // ...
+}
+
+function install_rational_package() {
+    // ...
+
+    function is_equal(x, y) {
+        return numer(x) * denom(y) === numer(y) * denom(x);
+    }
+
+    put("is_equal", list("rational", "rational"), is_equal);
+
+    // ...
+}
+
+function install_complex_package() {
+    // ...
+
+    function is_equal(z1, z2) {
+        return real_part(z1) === real_part(z2)
+               ? imag_part(z1) === imag_part(z2)
+               : false;
+    }
+
+    put("is_equal", list("complex", "complex"),
+            is_equal);
+
+    //...
+}
+	</JAVASCRIPT>
+      </SNIPPET>
+    </SOLUTION>
+</EXERCISE>
 
   <EXERCISE>
     Define a generic predicate