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To creates a standard library map, we first have to use the Map.Make functor to produce a module that is specialized for the type of keys we want. Type the following in utop:

module CharMap = Map.Make(Char);;

The output tells you that a new module named CharMap has been defined, and it gives you a signature for it. Find the values empty, add, and remove in that signature. Explain their types by your own words.