OCaml Programming: (Do not use List module or other library functions) diff: 'a list -> 'a list -> 'a list 

diff a b returns the difference of sets a and b in a. For example, eq (diff [1;3;2] [2;3]) [1] = true.

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let rec diff a b = (* YOUR CODE HERE *) 

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assert (eq (diff [1;3;2] [2;3]) [1]); assert (eq (diff ['a';'b';'c';'d'] ['a';'e';'i';'o';'u']) ['b';'c';'d']); assert (eq (diff ["hello";"ocaml"] ["hi";"python"]) ["hello";"ocaml"]);