def bitmask(self):
        """ A property holding a bitmask of the same length as this WireVector.
        Specifically it is an integer with a number of bits set to 1 equal to the
        bitwidth of the WireVector.

        It is often times useful to "mask" an integer such that it fits in the
        the number of bits of a WireVector.  As a convenience for this, the
        `bitmask` property is provided.  As an example, if there was a 3-bit
        WireVector `a`, a call to  `a.bitmask()` should return 0b111 or 0x7."""
        if "_bitmask" not in self.__dict__:
            self._bitmask = (1 << len(self)) - 1
        return self._bitmask