added purpose statement to expand-teaching-program

collects/lang/run-teaching-program.rkt

@@ -16,6 +16,10 @@
                                (symbol? boolean?)
                                any)])
 
+;; this function expands a port providing a program and a bunch of 
+;; arguments describing the user environment, and returns a thunk
+;; that returns the top-level expressions that make up the expanded
+;; program, one on each call.
 (define (expand-teaching-program port reader language-module teachpacks rep [module-name '#%htdp] [enable-testing? #t])
   (let ([state 'init]
         ;; state : 'init => 'require => 'done-or-exn