The state.dtd description is not enforced. This can lead to hard to diagnose
errors in the input file.
All edges using the same label must be in the same transition_set tag.
If a transition_set tag repeats a label used by an earlier transition_set tag,
it overwrites that transition_set. (Think of the labels as hash keys, I do.)
The bugs in UML::State apply here as well, since this script uses it
to make the pictures. See perldoc UML::State for those bugs.