Add missing extends statement to MDObjectPositionsSet

src/MDObjectPositionsSet.php

@@ -9,7 +9,7 @@
 /**
  * Class containing available positions on an object.
  */
-class MDObjectPositionsSet {
+class MDObjectPositionsSet extends MDValueSet {
 
     const POSITIONS = [
         'left', 'top_left', 'top', 'top_right', 'right', 'bottom_right', 'bottom', 'bottom_left', 'center', 'rear_side', 'edge', 'other',