_gr_ore_poly_shift_convert, _gr_ore_poly_shift_convert_difference

Rewrite op from src_alg to dst_alg, where both are among the builtin shift and difference algebras (corresponding to operators written in terms of the forward and backward shifts $S$, $S^{-1}$ and the forward and backward differences $S - 1$, $1 - S^{-1}$). The result satisfies $S^p \cdot \mathit{res} = \mathit{op}$. Here ctx is the common base ring and var is the index of the generator of ctx on which $S$ acts. Conversions that cross between the forward side ($S$, $S - 1$) and the backward side ($S^{-1}$, $1 - S^{-1}$) currently require a generic univariate polynomial base ring and otherwise return GR_UNABLE. The function returns an error status when the source or destination algebra is not of the required type. The output vector res has the same length len as op and must not alias it.

The second function is the specialisation to conversions between ORE_ALGEBRA_FORWARD_DIFFERENCE and ORE_ALGEBRA_BACKWARD_DIFFERENCE, the flag to_backward giving the direction.