fixes #823

Author: martin-henz

xml/chapter2/section5/subsection1.xml

@@ -947,6 +947,54 @@ function contents(datum) {
     arithmetic package.  This operation should work for ordinary numbers,
     rational numbers, and complex numbers.
     <LABEL NAME="ex:=zero?"/>
+    <SOLUTION>
+      <SNIPPET EVAL="no">
+	<JAVASCRIPT>
+// provided by GitHub user clean99
+
+function is_equal_to_zero(x) {
+    return apply_generic("is_equal_to_zero", list(x));
+}
+
+function install_javascript_number_package() {
+    // ...
+
+    put("is_equal_to_zero", "javascript_number",
+          x => x === 0);
+
+    // ...
+}
+
+function install_rational_package() {
+    // ...
+
+    function is_equal_to_zero(x) {
+        return numer(x) === 0;
+    }
+
+    put("is_equal_to_zero", "rational",
+           is_equal_to_zero);
+
+    // ...
+}
+
+function install_complex_package() {
+    // ...
+
+    function is_equal_to_zero(z) {
+        return real_part(z) === 0
+               ? imag_part(z) === 0
+               : false;
+    }
+
+    put("is_equal_to_zero", "complex",
+            is_equal_to_zero);
+
+    //...
+}
+	</JAVASCRIPT>
+      </SNIPPET>
+    </SOLUTION>
   </EXERCISE>
 
   <INDEX>generic arithmetic operations<CLOSE/></INDEX>