ilist iappend_destroy(ilist il1, ilist il2){
    if(!il1) return il2; // so the following "while" is correct
    int lengthToAdd = ilength(il2);  //   <- new
    ilist temp = il1;
    while(temp->rest) {
        temp->length+=lengthToAdd; //   <- new
        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;
}