model/program-structure.md

@@ -34,8 +34,8 @@ package action {
   Action <|.... Cancel
 
   interface Action {
-    + apply(game : Game) : Either<String, Game>
-    + getTimestamp() : LocalDateTime
+    + {abstract} apply(game : Game) : Either<String, Game>
+    + {abstract} getTimestamp() : LocalDateTime
   }
   note left
     A player action.