public PanelStackPopTransition(PanelStackSystem stack)
 {
     this.stack = stack;
 }
 public PanelStackPushTransition(PanelType panelType, PanelStackSystem stack, PanelViewControllerProvider provider)
 {
     this.panelType = panelType;
     this.provider  = provider;
     this.stack     = stack;
 }