ilist iappend(ilist il1, ilist il2){
    if(!il1) return il2; // so the following "while" is correct
    ilist temp = il1;
    while(temp->rest) temp = temp->rest;
    // now temp points to the last entry of il1
    while(il2) {
        temp->rest = il2;
        temp = temp->rest;
        il2 = il2->rest;
    }
    // now every node of il2 is appended to il1 and we just need to append the finishing null
    temp->rest = 0;
    return il1;
}
int main(void){
    ilist c = icons_destroy(4, icons_destroy(3, iempty()));
    ilist b = icons_destroy(1, icons_destroy(2, iempty()));
    ilist d = iappend_destroy (c, b);
    iprint("il", d);
    printf("%d", ilength(d)); // prints  4 3 1 2 in this case
    idelete(d);
}