Permutations

void permute(permtype * p, const char * expr) {
    p->perms[0][0] = expr[0];

    for (size_t i = 1; i < p->LEN; ++i)
        merge(p, i, expr[i]);

}

void merge(permtype * p, const size_t index, const char elem) {

    const size_t Nprev = index > 0 ? factorial(index) : 0;
    assert(p->N > Nprev);
    size_t j = Nprev;

    for (size_t i = 0; i < Nprev; ++i) {
        for ( size_t k = 0; k < index; ++k )
            splice(&(p->perms[j++]), p->perms[i], elem, k, index);

        p->perms[i][index] = elem;
    }

}