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Here is something that I think used to work in Lean 3, although I no longer have a working installation of Lean 3 with which to check that the details of this minimal example are precisely correct. I could define a function f like this:

section A

variable (x : Nat)
include x

def f (y : Nat) : Nat := y

end A

#check f

Because of the directive include x, the function f would actually have two arguments x and y, with x being present but unused. In other words, the type would be Nat -> Nat -> Nat.

This does not work in Lean 4: the line include x produces an error message unexpected identifier; expected command. Because the word "include" is used in so many different contexts, it is hard to search for information about this. Can anyone let me know whether there is a new syntax for this feature or whether it has been dropped?

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I believe include annd omit are still missing. There is some discussion, which you can read at the below links, about a possible better alternative include … in and omit … in, but those have not been implemented yet that I’m aware.

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