pop(i) or del a[i] is remove the item at i; insert(i, x) inserts x before the item at i; append(x) adds x after the last item (which can't be done with insert). remove(x) is unordered as it operates on the item not the index, so the opposite of remove should be an unordered add (which exists in an unordered set but not in a list). The rest of his points are more valid.