to allow productions E --> (E ;)* E
need to: distinguish ; from other symbols (operators?) that carry semantic meaning,
unless -, + : these are non-terminals with rusttype () and precedence 0.
This is not the same as identifying rusttype ().
-- but in some grammars [] have non-zero precedence
-- can also just assign precedence 1 to a symbol to give it meaning.
-- Then need to pre-process strtoks (...)* by intro another symbol, prior to
processing of *.