structure Env :> Env = struct
  type env = (string*int) list

  exception Env

  val empty = []

   fun update (a,b,[]) = [(a,b)]
     | update (a,b,(a1,b1)::t)
       = if a=a1 then (a,b)::t else (a1,b1)::(update (a,b,t))

   fun lookup (a,[]) = NONE
     | lookup (a,(a1,b1)::t)
       = if a=a1 then SOME b1 else lookup (a,t)

   fun toList t = t
end