'Returns the leading monomial of ``f``.
The Leading monomial signifies the monomial having
the highest power of the principal generator in the
expression f.
Examples
>>> from sympy import Poly
>>> from sympy.abc import x, y
>>> Poly(4*x**2 + 2*x*y**2 + x*y + 3*y, x, y).LM()
x**2*y**0'

def LM(f, order=None):

    return Monomial(f.monoms(order)[0], f.gens)
