MUPL(Made Up Programming Language)

variable
addition
integer
ifgreater
functions
pair
null
closures
lexical scope

(var  (string))                        ;; a variable, e.g., (var "foo")
(int  (num))                           ;; a constant number, e.g., (int 17)
(add  (e1 e2))                         ;; add two expressions
(ifgreater (e1 e2 e3 e4))              ;; if e1 > e2 then e3 else e4
(fun  (nameopt formal body))           ;; a recursive(?) 1-argument function
(call (funexp actual))                 ;; function call
(mlet (var e body))                    ;; a local binding (let var = e in body) 
(apair (e1 e2))                        ;; make a new pair
(fst  (e))                             ;; get first part of a pair
(snd  (e))                             ;; get second part of a pair
(aunit ())                             ;; unit value
(isaunit (e))                          ;; evaluate to 1 if e is unit else 0
(closure (env fun))                    ;;closures