'Leading monomial as a polynomial element.
Examples
>>> from sympy.polys.rings import ring
>>> from sympy.polys.domains import ZZ
>>> _, x, y = ring(\'x, y\', ZZ)
>>> (3*x*y + y**2).leading_monom()
x*y'

def leading_monom(self):

    p = self.ring.zero
    expv = self.leading_expv()
    if expv:
        p[expv] = self.ring.domain.one
    return p
