(msos IDENTIFIERS is
 Id .
 Env = (Id, Int) Map .
 Exp ::= let Id = Int in Exp end
       | Id .
 Label = { env : Env, ... } .

    Env' := (Id |-> Int) / Env,
                       Exp -{env = Env', ...}-> Exp'
 -- ------------------------------------------------
 (let Id = Int in Exp end) : Exp -{env = Env, ...}-> 
   (let Id = Int in Exp' end) .

 (let Id = Int in Int' end) : Exp --> Int' .

    Int := lookup (Id, Env)
 -- --------------------------
 Id : Exp -{env = Env, -}-> Int .
sosm)
