'Convert a monomial instance to a SymPy expression.'

def as_expr(self, *gens):

    gens = (gens or self.gens)
    if (not gens):
        raise ValueError(("can't    convert    %s    to    an    expression    without    generators" % self))
    return Mul(*[(gen ** exp) for (gen, exp) in zip(gens, self.exponents)])
