diff --git a/networks.rkt b/networks.rkt
index 179dcd8..3787ce2 100644
--- a/networks.rkt
+++ b/networks.rkt
@@ -184,6 +184,10 @@
 ;;; A network consists of a mapping from its variables to its update
 ;;; variables, as a well as of a mapping from its variables to
 ;;; their domains.
+;;;
+;;; The domain mapping does not have to assign domains to all
+;;; variables (e.g., it may be empty), but in this case the functions
+;;; which need to know the domains will not work.
 (struct network (functions domains))
 
 ;;; Builds a network from a given hash table assigning functions to
@@ -261,6 +265,10 @@
 ;;; A network form consists of a mapping from variables to the forms
 ;;; of their update functions, together with a mapping from its
 ;;; variables to its update functions.
+;;;
+;;; The domain mapping does not have to assign domains to all
+;;; variables (e.g., it may be empty), but in this case the functions
+;;; which need to know the domains will not work.
 (struct network-form (forms domains))
 
 ;;; Build an update function from an update function form.