lemenations for the <code>add()</code>,
<code>subtract()</code> and other methods declared by the Arithmetic
interface.
<blockquote>
<pre>
class ImaginaryNumber extends Number implements Arithmetic {
    . . .
    public Number add(Number) {
	. . .
    }
    public Number subtract(Number) {
	. . .
    }
}
</pre>
</blockquote>
Note that the method signatures of the methods declared in the Arithmetic
interface and the method signatures of the methods implemented in the
ImaginaryNumber class <em>must match</em