Considering potential ambiguity, I suggest `d1.append(d2)` so we can have an additional argument saying `d1.append(d2, mode="some mode that tells how this function behaviours")`.

If we are really to have the new syntax `d1 + d2`, I suggest leaving it for `d1.append(d2, mode="strict")` which raises an error when there're duplicate keys. The semantics is nature and clear when two dicts have no overlapping keys.