Lack of documentation on the preconditions to apply existing transformations

[rcasjimenez]

According to #199, it would be good to add information about the requirements (preconditions) necessary to apply each of the existing transformations (pivot, lcomp, bialgebra and so on). There is no information in :

• rules.py
• simplify.py
• hrules
• hsimplify

No reference to a graph-like condition or any other prerequisite is mentioned.