(added a note)

firmware/lib/layout/layer-stack.h

@@ -11,6 +11,17 @@
  *
  * This file is meant to be included and used by the keyboard layout
  * implementation.
+ *
+ * Notes:
+ * - This interface and implementation, perhaps contrary to good programming
+ *   practice, define a single (private) data structure *and* the (public)
+ *   operations on it, such that one cannot have multiple layer stacks operated
+ *   on by these functions.  It would, of course, be possible to split the
+ *   functions and their data structure apart, but 1) that would make the
+ *   functions slightly more complicated, and 2) anything else that needs a
+ *   stack should probably be using ".../firmware/lib/data-types/list.h".  So
+ *   I'm leaving it as is for now.  I may reconsider at some point in the
+ *   future.
  */