add VISB_SVG_EVENTS definition to button_def

Signed-off-by: Michael Leuschel <leuschel@uni-duesseldorf.de>

Button/button_def.mch

@@ -8,12 +8,13 @@ DEFINITIONS
   VISB_SVG_UPDATES == rec(`id`:"button",
      fill: IF button=TRUE THEN "green" ELSE "red" END);
   VISB_SVG_HOVERS == rec(`id`:"button",
-     stroke:"gray", `stroke-width`:"5")
+     stroke:"gray", `stroke-width`:"5");
+  VISB_SVG_EVENTS == rec(`id`:"button", event:"toggle_button", predicate:"btrue");
 VARIABLES button
 INVARIANT button:BOOL
 INITIALISATION button := FALSE
 OPERATIONS
-  toggle_button = BEGIN
+  toggle_button /* desc change status of button */ = BEGIN
     button:= bool(button=FALSE)
   END
 END
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