[Kirill Balunov] It may be worth taking a look at + and +=. However, the semantic difference is due to the dunder add and dunder iadd methods- necessary for supporting both mutable and immutable sequences. See my earlier mail for discussion on this topic. By the way,

the absence of literals for `set` and `frozenset` types,

The literal for the set is {1, 2, 3, 4}.