se that extractors are assumed to
return the same key when passed the same element: this is in consonance
with the informal understanding that keys are actually some "part"
of the element and do not depend on external data.
</p>

<h3><a name="read_write_key_extractors">Read/write key extractors</a></h3>

<p>
A key extractor is called <i>read/write</i> if it returns a non-constant reference
to the key when passed a non-constant element, and it is called <i>read-only</i>
otherwise. Boost.MultiIndex requires t