diff --git a/src/line_gap.sml b/src/line_gap.sml
index 2a20d82..8d4cbaa 100644
--- a/src/line_gap.sml
+++ b/src/line_gap.sml
@@ -28,6 +28,8 @@ sig
   val goToIdx: int * t -> t
   val goToLine: int * t -> t
 
+  val getLineNumberOfIdx: int * t -> int
+
   (* for testing *)
   val verifyIndex: t -> unit
   val verifyLines: t -> unit