'Return True if all lexical rules are "preterminals", that is,
unary rules which can be separated in a preprocessing step.
This means that all productions are of the forms
A -> B1 ... Bn (n>=0), or A -> "s".
Note: is_lexical() and is_nonlexical() are not opposites.
There are grammars which are neither, and grammars which are both.'

def is_nonlexical(self):

    return self._is_nonlexical
