code: def bitmask(self):
          
          if "_bitmask" not in self.__dict__:
              self._bitmask = (1 << len(self)) - 1
          return self._bitmask
docs: 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.