Base Formula on Counter instead of dict. This allows some multiset operators to be defined more efficiently. The operators __and__ and __sub__ are added.
Add a ParseError exception in the formula module and use it to report errors in Formula.parse(). The parse exception may contain a span indicating the location of the parse error in the input string. This information is used in balancecheck to show better error messages.
FormulaonCounterinstead ofdict. This allows some multiset operators to be defined more efficiently. The operators__and__and__sub__are added.ParseErrorexception in theformulamodule and use it to report errors inFormula.parse(). The parse exception may contain a span indicating the location of the parse error in the input string. This information is used inbalancecheckto show better error messages.