Include better map type

[jogibear9988]

see https://www.youtube.com/watch?v=i21l5MSZ3wQ

[uhyo]

Can I have text explaining the improvement rather than a video?

I just watched ~30 seconds; if it is about making Map#has a type guard, we need to consider what'd happen if delete is called after a has guard.

[jogibear9988]

Yeah, thought cause of the "has" type guards.