Semantic evaluation function
let rec sem (e:exp) (r:env) = match e with
| Eint(n) -> alfa(Int(n))
| Var(i) -> applyenv(r,i)
| Sum(a,b) -> plus ( (sem a r), (sem b r))
| Diff(a,b) -> diff ( (sem a r), (sem b r))
| Ifthenelse(a,b,c) -> let a1 = sem a r in
(if valid(a1) then sem b r else
(if unsatisfiable(a1) then sem c r
else merge(a1,sem b r,sem c r)))
| Fun(ii,aa) -> makefun(ii,aa,r)
| Rec(i,e1) -> makefunrec (i,e1,r)
| Appl(a,b) -> applyfun(sem a r, sem b r)
| Let(i,e1,e2) -> let d = sem e1 r in if d = alfa(Wrong) then d else
| Letrec(i,e1,e2) -> sem (Let(i,Rec(i,e1),e2)) r
| Letmutrec ((i1,e1),(i2,e2),e3) ->
sem e3 (makemutrec((i1,e1),(i2,e2),r))