ForbiddenSequence for non-path sequences

[Whebon]

The ForbiddenSequence constraint forbids a sequence of rules in a vertical path. However, most grammars have a different notion of a 'sequence'. For example, the sequence of operations in the robot grammar do not lie on a path.

To forbid the sequence [right, left], we either need to:

1. rewrite the grammar.
2. change the semantics of ForbiddenSequence.
3. make a new forbidden sequence constraints for a different kind of sequence.