diff --git a/02-trees/main.rkt b/02-trees/main.rkt
index baee183..d4c28bf 100644
--- a/02-trees/main.rkt
+++ b/02-trees/main.rkt
@@ -21,13 +21,15 @@
 ; Tree t -> Bool
 ; Determines whether a Tree is a binary search tree.
 (define (bst? t)
-  (local [; Convenient form of greater than.
+  (local [; <2: U{Num, Sym}, Num, U{Num, Sym} → Bool
+          ; Convenient form of greater than. Slightly hack.
           (define (<2 a b c)
             (cond [(and (symbol? a) (symbol? c)) #t]
                   [(symbol? a) (< (- b 1) b c)]
                   [(symbol? c) (< a b (+ b 1))]
                   [else (< a b c)]))
 
+          ; recurse: t: Tree, lower: Num, upper: Num → Bool
           ; Recursively checks left & right with lower & upper bounds.
           (define (recurse t lower upper)
             (cond [(symbol? t) (symbol=? t 'end)]