def has(self, state):
return not self.unpack(state).is_nothing()<EOL>
Returns `True` when the state would be successfully focused
        by this prism, otherwise `False`.

            >>> from lenses.maybe import Nothing, Just
            >>> def pack(focus):
            ...     return focus
            >>> def unpack(state):
            ...     if state > 0:
            ...         return Just(state)
            ...     return Nothing()
            >>> positive = Prism(unpack, pack)
            >>> positive.has(-1)
            False
            >>> positive.has(1)
            True