http://graphml.graphdrawing.org/xmlns/graphml element default
Documentation
Description: In GraphML there may be data-functions attached
to graphs, nodes, ports, edges, hyperedges and
endpoint and to the whole collection of
graphs described by the content of <graphml>.
These functions are declared by <key> elements
(children of <graphml>) and defined by <data>
elements.
The (optional) <default> child of <key> gives
the default value for the corresponding function.
Occurence: <key>.