Implement symbolic transitions.

[marcofavorito]

Is your feature request related to a problem? Please describe.

With the current implementation, all the transitions must be stated explicitly and with the alphabet as arc labels.

Describe the solution you'd like

A way to specify a broader set of possible transitions expressed by propositional formulas, e.g. true to specify any symbol (even not present in the alphabet), false the opposite, a to indicate that the proposition a must be true.

Describe alternatives you've considered

Additional context

[marcofavorito]

#72 closes this issue.