d as {{"a", "b"}: {1, 2}} = {"a": 1}
dict = !d

dict.insert! "b", 2
_ = dict.get("a") == "a" # ERR
_ = dict.get("b") == "a" # ERR
_ = dict.get("c") # OK
_ = dict["b"] # OK
_ = dict["c"] # ERR

_: Mapping(Int, Str) = !{1: "a", 2: "b"}
_: Mapping(Str, Int) = !{1: "a", 2: "b"} # ERR

Manager = Class {
    .tasks = List Dict! { Int: Str }
}
Manager.
    show_tasks! self, id: Str =
        for! self.tasks, task =>
            print! task[id] # ERR