Computer Science about Functional language FL expression / Type systems

You have time, I'd really appreciate it if you could give me the specific answer.

you have time, I'd really appreciate it if you could give me the specific answer. Functional language FL expression afgx. f(gxx) Find the principal type of an expression in the functional language FL by the technique of type reconstruction (a method of constructing a proof diagram of a type, forming a system of equations for the type, and solving it by a unification algorithm). (Hint: The principal type is (O O) (O O O) O. However, not all the same type variables are included in O on the left. Note that the type configuration Always note that the child is right-joined, that is, o O is O) and not (O O) O.) FL type FL small-step semantics y![dus MM (app) MNM'N (app2) (ax.MN - MXN] M M (app3) op Mop M' (if op Op) NN (app) (if op Op) op CN-op CN (Basic type) (Basic type) (List type) (functional type) () () ti:= int | bool | list(T) | T +7 (app5) (if op"*72**C,"CZ"? = c) op C1 C2 - (case1) LL' case L of nil = M|x :: y = N case L' of nil Mx y = N LL if L then M else N if L'then M else N (if) (if 2) if truebool then M else NM (case2) case nil of nil = Mxy = N - M (case 3) (if3) if falsebool then M else NN (let) let x = N in MMXN] case L1 :: L2 of nil = M*: y = N N[x - Lilly L2]