Suggestion: make sequence and map interfaces more similar

[Paul Rubin]

Steven D'Aprano <steve at pearwood.info> writes:
> I want to see an actual application where adding a new key to a
> mapping is expected to change the other keys.

directory["mary.roommate"] = "bob"
directory["mary.address"] = None   # unknown address
...
directory["bob.address"] = "132 Elm Street"

Since Bob and Mary are roommates, they have the same address, so the
application might want to update both addresses once it learns one of
them.