assignment_collection._SymbolGenerator: Potential for generating non-unique symbols
>>> from sumpy.assignment_collection import _SymbolGenerator
>>> sg = _SymbolGenerator(set())
>>> sg("s_0")
s_0
>>> sg("s")
s
>>> sg("s")
s_0
>>> from sumpy.assignment_collection import _SymbolGenerator
>>> sg = _SymbolGenerator(set())
>>> sg("s_0")
s_0
>>> sg("s")
s
>>> sg("s")
s_0