<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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