 variable 
number of elements. It's somewhere between <a
href="associative_property_map.html">associative_property_map</a> and 
<a href="iterator_property_map.html">iterator_property_map</a>. The latter
is very fast, but requires that the number of stored elements is known
when creating property map. The former does not have this requirement, but
is slower, and requires stored elements to be comparable.

<p>
The <tt>vector_property_map</tt> uses mapping from key to indices,
and allows to add new elements. I