Even right now you may have Regular Expressions that have follow restrictions. Even Ids have follow restrictions.
When you write
syntax Id ::= Token{[a-zA-Z][a-zA-Z0-9]*}
The front end will generate automatically a follow restriction (lookahead) that checks to see if the current location is not followed by any of the [a-zA-Z0-9].

The problem in this location is when you have such a lookahead for an empty string: "" -/- [a-z]
Something like that.