Fields:
  preferencesdlg_exists = [True]
  preferencesdlg_visible = [False, True]
Conditions:
Limits:
Initial: dialog_preferences
  State: preferencesdlg_exists=False, preferencesdlg_visible=False
         preferencesdlg_exists=True, preferencesdlg_visible=True
Transition: action_preferences
  State: 
         preferencesdlg_visible=True
Transition: dialog_preferences_close
  State: 
         preferencesdlg_visible=False
