ence to the property value (a const reference if
the map is not mutable).

<P>
Similar to the <TT>iterator_traits</TT> class of the STL, there is a
<TT>property_traits</TT> class that can be used to deduce the types
associated with a property map type: the key and value types, and
the property map category (which is used to tell whether the
map is readable, writeable, or both). In the <TT>relax()</TT>
function we could have used <TT>property_traits</TT> to declare local
variables of the distance property ty