diff --git a/facility.go b/facility.go
index a4a4ab9d5..27e0dc9a5 100644
--- a/facility.go
+++ b/facility.go
@@ -31,6 +31,15 @@ type Facility interface {
 	With(...Field) Facility
 	Enabled(Entry) bool
 	Log(Entry, ...Field) error
+
+	// Log does not call Enabled
+	// Enabled composes like "any"
+
+	// Enabled may be non-deterministic.
+	// Log may drop the entry, but must do so deterministically.
+	// Log must duplicate an deterministic parts of Enabled, otherwise you
+	// won't work correctly under Tee!
+
 }
 
 // WriterFacility creates a facility that writes logs to an io.Writer. By