Needed a default of no Modifiers to click_on since that's a very common operation. Also a constant to represent that.

include/guecs/ui.hpp

@@ -26,13 +26,16 @@ namespace guecs {
     };
 
   enum class ModBit {
-    hover=0,
+    NONE=0,
-    left=1,
+    hover=1,
-    right=2
+    left=2,
+    right=3
   };
 
   using Modifiers = std::bitset<16>;
 
+  constexpr const Modifiers NO_MODS{size_t(ModBit::NONE)};
+
   struct Clickable {
     /* This is actually called by UI::mouse and passed the entity ID of the
      * button pressed so you can interact with it in the event handler.
@@ -74,7 +77,7 @@ namespace guecs {
       void init();
       void render(sf::RenderWindow& window);
       bool mouse(float x, float y, Modifiers mods);
-      void click_on(Entity slot_id, Modifiers mods);
+      void click_on(Entity slot_id, Modifiers mods=NO_MODS);
       void debug_layout(sf::RenderWindow& window);
 
       Entity entity() { return ++entity_count; }