'Leading monomial tuple according to the monomial ordering.
Examples
>>> from sympy.polys.rings import ring
>>> from sympy.polys.domains import ZZ
>>> _, x, y, z = ring(\'x, y, z\', ZZ)
>>> p = x**4 + x**3*y + x**2*z**2 + z**7
>>> p.leading_expv()
(4, 0, 0)'

def leading_expv(self):

    if self:
        return self.ring.leading_expv(self)
    else:
        return None
