_flint_mpn_poly_mulmid_classical

Sets (\mathit{res}, (\mathit{nhi} - \mathit{nlo}) s) to the coefficients [\mathit{nlo}, \mathit{nhi}) of the product of (f, \mathit{flen}\, n_1) and (g, \mathit{glen}\, n_2), viewed as polynomials with n_1-limb respectively n_2-limb coefficients, computed modulo B^s by the classical algorithm. Requires n_1 \ge n_2 and s \in \{n_1 + n_2 - 1, n_1 + n_2, n_1 + n_2 + 1\}. The coefficients are unsigned for sgn = 0, two's complement for sgn = 1, and sign-magnitude for sgn = 2 (a sign limb followed by n_1 respectively n_2 magnitude limbs; the output is two's complement). The results are exact provided that the requested coefficients fit in s limbs. Squaring is detected when f == g and flen == glen.