Contraction of transitions

Contraction of Petri net or STG transitions is apparently a tricky business. There are 4 levels of "contractability" of a transition with corresponding conditions:

0) Contraction is impossible:
  - Transition has a read-arc or self-loop.
  - Weighted arcs are required for the contraction (currently unsupported).

1) Contraction is possible, but language can change:
  - Transition has no read-arcs or self-loops.

2) Contraction is possible, language is preserved, but safeness can be violated:
  - Type-1-secure: There are no choice places in the preset (preset can be empty).
  - Type-2-secure: There is at least one unmarked place in the postset AND there are no merge places in the postset (the postset cannot be empty).

3) Contraction is possible, language is preserved, safeness is preserved:
  - Type-1 safe: The only place in the postset is unmarked AND it is not a merge.
  - Type-2 safe: There is a single place in the preset AND all the postset places are unmarked and not merge places (the postset cannot be empty).
  - Type-3 safe: The only preset place is not a choice.