Rather than using tuples for set belonging, use a frozen set. That eliminates the need to separate them by lengths.
Also, 'Pop' is not a reserved word. It's perfectly valid as a possible substitution identifier, so allow it. If used, it will go to the used words anyway and thus will be skipped by the sequential name generator.
Identifiers can be equal if they belong to different syntactic scopes. That will allow better reuse and less creation of new identifiers, most notably as function and event parameter names.
The implementation would require a stack of counters where the current value is pushed when entering a new scope, and popped when exiting, rather than using a single counter for the whole program.
