sary for efficient updates of the
heap data structure when an edge is relaxed. The type
<tt>VertexIndex1Map</tt> must be a model of <a
href="../../property_map/doc/ReadablePropertyMap.html">Readable Property
Map</a>. The value type of the map must be an integer type. The vertex
descriptor type of graph 1 needs to be usable as the key type of the
map.<br>
<b>Default:</b> <tt>get(vertex_index, g1)</tt>
    Note: if you use this default, make sure your graph has
    an internal <tt>vertex_index</tt> property. 