'Return True if self is a logical operand.
For being logical, it must be a finite number with a sign of 0,
an exponent of 0, and a coefficient whose digits must all be
either 0 or 1.'

def _islogical(self):

    if ((self._sign != 0) or (self._exp != 0)):
        return False
    for dig in self._int:
        if (dig not in '01'):
            return False
    return True
