All functors of the sol-library and all CoSy-fSDL-functors incooperate
print routines which print a graph in VCG format to a file. You may
call these routines in a target predicate to look at the current shape
of a graph. You have to supply a function which provides a text
label for each node. This function takes a node and returns a label
string which is printed in the file.
E.g. for a hgraph this works as follows:
