add missing implementation of Cursor.matchPair when strIdx is in tl

fcore/cursor.sml

@@ -1632,18 +1632,7 @@ struct
                 )
           end
 
-  fun matchPair (lineGap: LineGap.t, cursorIdx) =
-    let
-      val {rightStrings, idx = bufferIdx, leftStrings, ...} = lineGap
-    in
-      case rightStrings of
-        shd :: stl =>
-          let
-            (* convert absolute cursorIdx to idx relative to hd string *)
-            val strIdx = cursorIdx - bufferIdx
-          in
-            if strIdx < String.size shd then
-              (* strIdx is in this string *)
+  fun startMatchPair (strIdx, shd, stl, cursorIdx) =
     let
       val chr = String.sub (shd, strIdx)
     in
@@ -1690,6 +1679,20 @@ struct
             )
        | _ => cursorIdx)
     end
+
+  fun matchPair (lineGap: LineGap.t, cursorIdx) =
+    let
+      val {rightStrings, idx = bufferIdx, leftStrings, ...} = lineGap
+    in
+      case rightStrings of
+        shd :: stl =>
+          let
+            (* convert absolute cursorIdx to idx relative to hd string *)
+            val strIdx = cursorIdx - bufferIdx
+          in
+            if strIdx < String.size shd then
+              (* strIdx is in this string *)
+              startMatchPair (strIdx, shd, stl, cursorIdx)
             else
               (* strIdx is in tl *)
               (case stl of
@@ -1698,7 +1701,7 @@ struct
                      val strIdx = strIdx - String.size shd
                      val leftStrings = shd :: leftStrings
                    in 
-                     (print "match pair throw err\n"; raise Match)
+                      startMatchPair (strIdx, stlhd, leftStrings, cursorIdx)
                    end
                | [] => cursorIdx)
           end