add example comment

kaitai/expr.rkt

@@ -120,10 +120,20 @@
            [(lparen exp rparen) $2])
       (apply-args
         [(exp) (list $1)]
-        [(exp comma apply-args) (cons $1 $3)])
-      ]))
+        [(exp comma apply-args) (cons $1 $3)])]))
+
+;; kaitai expr AST
+;; it's like racket mostly. builtins are
+;; - all the builtin operators
+;; - (apply func args ...): applies a function
+;; - (array args ...): constructs array
+;; - (get obj attr): references an attribute on an object
+;; primitive types: boolean, number, string, symbol (identifier)
 
 (define test2 "true and 'a' != 'b' ? -1 + ~bits : ('hello' + 'world').substring(2, 3)")
+;; => '(if (and #t (!= "a" "b"))
+;;         (+ -1 (~ bits))
+;;         (apply (get (+ "hello" "world") substring) 2 3))
 
 (let ([input (open-input-string test2)])
   (kaitai-parser (lambda () (kaitai-lexer input))))