I was referring to __setitem__: you end up with expressions like `m[...] = ...` where practically speaking the meaning of `=` is no longer obvious. In some contexts it can end up meaning pretty unintuitive things (think pandas view into dataframes etc.). But, yeah, most languages have something like this since it's pretty useful, and only way to get away from these ambiguities is using purely immutable data which can be a no-go performance wise when you're working with references to multi-gigabyte structures...