let fact x = ImpProg(
  Dseq(Const ("n", x),Dseq(Var "i",Var "f")),
  Cseq(Cseq(AssignVar ("i", N 1),AssignVar ("f", N 1)),
       While(Or(Lt (Val "i", Val "n"),Eq (Val "i", Val "n")),
             Cseq(AssignVar ("f", Mul (Val "f",Val "i")),
                  AssignVar ("i", Add (Val "i",N 1))))));;