[gnome-sudoku] DifficultyCategory: use assert_not_reached()

[Michael Catanzaro <mcatanzaro src gnome org>]

commit a895382659d1e88a31d21c39ebac6d553f7e8220
Author: Michael Catanzaro <mcatanzaro gnome org>
Date:   Mon Jun 2 22:15:48 2014 -0500

    DifficultyCategory: use assert_not_reached()

 src/sudoku-solver.vala |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/sudoku-solver.vala b/src/sudoku-solver.vala
index 03d025d..59ef3a7 100644
--- a/src/sudoku-solver.vala
+++ b/src/sudoku-solver.vala
@@ -747,7 +747,7 @@ public enum DifficultyCatagory {
             case VERY_HARD:
                 return _("Very Hard Difficulty");
             default:
-                return _("Undefined");
+                assert_not_reached ();
         }
     }
 }