'Create a Poly instance from ``self``.'

def as_poly(self, x=None):

    from sympy import Dummy, Poly, PurePoly
    if (x is not None):
        return Poly.new(self.rep, x)
    elif (self.alias is not None):
        return Poly.new(self.rep, self.alias)
    else:
        return PurePoly.new(self.rep, Dummy('x'))
