another setter

@equips_to.setter
# No default on slot so the caller MUST supply something.
# Default of the equippable flag is True, because the
# caller will eithe rwant to set or clear that status.
def equips_to(self, slot, is_equippable=True):
    # assume self._equips_to is already initialized to
    # a valid set by being loaded, or set to an empty
    # set in __init__.
    if type(slot) is set:
        if is_equippable:
            # When will self ever NOT be an item?
            #if self.is_item:
            self._equips_to |= slot
        else:
            self._equips_to -= slot
    elif type(slot) is str:
        if slot in equips_to_strings.keys():
            if is_equippable:
                self._equips_to.add(slot)
            else:
                self._equips_to.discard(slot)
        else:
            raise ValueError('Invalid slot name %r for %r' % (slot, self))
    else:
        raise TypeError('slot must be a set or a string, not %r' % type(slot))