Optional delegate in charge of History related events. Provides nice isolation for unit testing, and allows pre- or post-processing of tokens. Methods correspond to the like named methods on {@link History}.

abstract class Historian {
 /**
  * Adds a {@link com.google.gwt.event.logical.shared.ValueChangeEvent}
  * handler to be informed of changes to the browser's history stack.
  * 
  * @param valueChangeHandler the handler
  * @return the registration used to remove this value change handler
  */
 HandlerRegistration addValueChangeHandler(ValueChangeHandler<String> valueChangeHandler);

 /**
  * @return the current history token.
  */
 String getToken();

 /**
  * Adds a new browser history entry. Calling this method will cause
  * {@link ValueChangeHandler#onValueChange(com.google.gwt.event.logical.shared.ValueChangeEvent)}
  * to be called as well.
  */
 void newItem(String token, bool issueEvent);
}