diff --git a/src/mark_matrix.rs b/src/mark_matrix.rs
index 2aa0d75..a6cc261 100644
--- a/src/mark_matrix.rs
+++ b/src/mark_matrix.rs
@@ -32,7 +32,7 @@ impl MarkMatrix {
         }
     }
 
-    pub fn unmark(&mut self, pos: (usize, usize)) {
+    fn unmark(&mut self, pos: (usize, usize)) {
         self.marks[pos] = Mark::None;
     }