diff --git a/object/transformer/fmt.go b/object/transformer/fmt.go
deleted file mode 100644
index 193b8f4..0000000
--- a/object/transformer/fmt.go
+++ /dev/null
@@ -1,5 +0,0 @@
-package transformer
-
-type EpochSource interface {
-	CurrentEpoch() uint64
-}
diff --git a/object/transformer/types.go b/object/transformer/types.go
index 0525eb0..f030e26 100644
--- a/object/transformer/types.go
+++ b/object/transformer/types.go
@@ -16,6 +16,11 @@ type AccessIdentifiers struct {
 	ParentHeader *object.Object
 }
 
+// EpochSource is a source for the current epoch.
+type EpochSource interface {
+	CurrentEpoch() uint64
+}
+
 // ObjectTarget is an interface of the object writer.
 type ObjectTarget interface {
 	// WriteHeader writes object header w/ payload part.