of a composite key:
</p>

<blockquote><pre>
<span class=comment>// try to locate streets in quadrants with x==0
// compile-time error: hashed indices do not allow such operations</span>
<span class=identifier>std</span><span class=special>::</span><span class=identifier>pair</span><span class=special>&lt;</span><span class=identifier>street_locator</span><span class=special>::</span><span class=identifier>iterator</span><span class=special>,</span><span class=identifier>street_locator</span><span class=spec