Updated "help" statement of pn_interface

integration/code/pn_interface.alc

@@ -122,7 +122,8 @@ Element function petrinet_loaded(model : Element):
 			output("Generic model operations:")
 			output("  instantiate -- Create a new model element")
 			output("  delete      -- Delete an existing element")
-			output("  attr_add    -- Add an attribute to an existing element")
+			output("  attr_add    -- Add an attribute to an element")
+			output("  attr_del    -- Delete an attribute of an element")
 			output("  rename      -- Rename an existing element")
 			output("  modify      -- Modify the attributes of an element")
 			output("  list        -- Prints the list of elements in the model")