Commented invoke() usage.

chapter3/interpreter.coffee

@@ -175,7 +175,13 @@ evaluate = (e, env) ->
         when "begin" then eprogn (cdr exp), env
         when "set!" then update (nvalu cadr exp), env, (evaluate (caddr exp), env)
         when "lambda" then make_function (astSymbolsToLispSymbols cadr exp), (cddr exp), env
-        else invoke (evaluate (car exp), env), (evlis (cdr exp), env)
+            
+        else
+            # Note that invoke ultimately resolves to a (native)
+            # function generated by make_function, and a (interpreter)
+            # list that the generated (native) function knows how to
+            # unpack into the actual (native) operation.
+            invoke (evaluate (car exp), env), (evlis (cdr exp), env)
     else
       invoke (evaluate (car exp), env), (evlis (cdr exp), env)
   else