Factor out coefficients of all factors.

def factor_coeff(cls, ops, kwargs):
    """Factor out coefficients of all factors."""
    coeffs, nops = zip(*map(_coeff_term, ops))
    coeff = 1
    for c in coeffs:
        coeff *= c
    if coeff == 1:
        return nops, coeffs
    else:
        return coeff * cls.create(*nops, **kwargs)