1 files changed, 3 insertions, 0 deletions

diff --git a/src/Sequence/Formula.hs b/src/Sequence/Formula.hs
index 9000c58..d486bab 100644
--- a/src/Sequence/Formula.hs
+++ b/src/Sequence/Formula.hs
@@ -46,6 +46,9 @@ instance a :<: (a, b) where
 instance a :<: (b, a) where
   ctx' = _2
 
+instance () :<: a where
+  ctx' = united
+
 data Context small = forall large. (small :<: large) => Context large
 
 ctx :: Traversal' (Context input) input