Applies a^(m+n) = a^m * a^n, a^0 = 1, a^1 = a.
We do normalize a^0 and a^1 when converting expressions into polynomials,
but we need to normalize them here when for other preprocessing steps such as
a / b = a*b⁻¹. If b is of the form c^1, it will be treated as an
atom in the comm ring module.