diff --git a/networks.rkt b/networks.rkt
index 8d0bc8d..12a5ce0 100644
--- a/networks.rkt
+++ b/networks.rkt
@@ -43,6 +43,7 @@
                [state? (-> any/c boolean?)]
                [update-function-form? (-> any/c boolean?)]
                [network-form? (-> any/c boolean?)]
+               [modality? (-> any/c boolean?)]
                [mode? (-> any/c boolean?)])
  ;; Contracts
  (contract-out [state/c contract?]
@@ -256,8 +257,11 @@
 ;;; This section contains definitions for building and analysing the
 ;;; dynamics of networks.
 
-;;; A mode is a set of sets of variables.
-(define mode? (set/c (set/c variable?)))
+;;; A modality is a set of variable.
+(define modality? (set/c variable?))
+
+;;; A mode is a set of modalities.
+(define mode? (set/c modality?))
 
 ;;; A network dynamics is a network plus a mode.
 (struct dynamics (network mode)