Add ot remove tokens from a set according to a spec string.
The spec string is a sequence of tokens, with each group optionally prefixed
by one of the operators '=', '+', or '-'.
The '=' operator resets the set to contain only the following token(s).
Add ot remove tokens from a set according to a spec string. The spec string is a sequence of tokens, with each group optionally prefixed by one of the operators '=', '+', or '-'. The '=' operator resets the set to contain only the following token(s).