sf 24.01.2006:
   Need to improve toGXL so that attributes are properly written out.

wh 28.01.2005: can we have a "==" function for graphs (e.g. graphNEL)?

sf 24.08.2005: 
    move methods into file named for the generic.

    document all generics and all classes

    document all methods within class doc.

    each graph representation class should have its own validation
    function.  This way, adding a new representation doesn't require
    adding another if statement into a monolithic validation routine.

    move methods up into the graph class where possible.

