Set.._add() and _remove():
- no longer test anti-state before calling _add_value() and _remove_value()
- allows set to output values that do not contain input values, which is what is needed most of the time
- if one wants to manage an anti-state on input values, one will now have to use an explicit set() before
- this also makes _add() and _remove() faster by processing the anti-state only once