 the
corresponding key extractor can be used when passed constant elements (this
relates to the elements specified in the first column, not the referenced
<code>T</code> objects). The only negative cases are for <code>T::g</code> and
<code>T:gg</code> when the elements are raw <code>T</code> objects, which make sense
as we are dealing with a non-constant member function (<code>T::g</code>)
and a function taking <code>T</code> by
non-constant reference: this also implies that <code>multi_index_container</cod