structure Environment : EnvironmentSig =
struct
    exception Lookup
    datatype Environment = Env of (string*int) list

    val empty = Env []
        
    fun lookup (Env((k,e)::t)) key = if (k = key) then e
                                     else lookup (Env t) key
      | lookup _ _ = raise Lookup

    fun enter (entry, Env entries) = Env(entry :: entries)
end;