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+=== to add a new widget ===
+
+
+=== to add a new widget option ===
+- add a new variable		<widget_option.h>
+- to initialize the variable, modify _wod_init()		<widget_option.c>
+- add a new entry in widget_option_content_update()		<widget_option.c>
+	ex) WIDGET_OPTION(notify, ETV_ID_NOTIFY)
+- add specific options		<widget_option.c>
+	- check
+		- for caller
+			WIDGET_OPTION_CHECK_ADD("Allow background events", wod->notify_event, _notify_event_changed_cb);
+		- for callee
+			WIDGET_OPTION_CHECK_CHANGED_CB(notify_event, wod->notify_event);
+- use specific options		<widget.c>
+	if (wod)
+		{
+			if (wod->btn_text)
+				elm_object_text_set(o, "This is a Button");
+		}