mpi_sub_ui
ret = mpi_sub_ui(pone, ctx->p, 1);
if (mpi_sub_ui(n1, n, 1) || mpi_cmp(x, n1) >= 0) {
int mpi_sub_ui(MPI w, MPI u, unsigned long vval);
EXPORT_SYMBOL_GPL(mpi_sub_ui);