+-- | Is the given SigDef a FApp?
+is_FApp :: SigDef -> Bool
+is_FApp d = case d of
+ (FApp _ _ _) -> True
+ _ -> False
+
+-- | Which signals are used by the given SigDef?
+sigDefUses :: SigDef -> [SignalId]
+sigDefUses (UncondDef (Left id) _) = [id]
+sigDefUses (UncondDef (Right expr) _) = sigExprUses expr
+sigDefUses (CondDef cond true false _) = [cond, true, false]
+sigDefUses (FApp _ args _) = concat $ map Foldable.toList args
+