code: public static Progression sub(Progression model, int extent) {
  		if (model != null) {
  			return model.subTask(extent);
  		}
  		return null;
  	}
docs: Create and replies a task progression model that is for
  a subtask of the task associated to the given progression model.
  
  <p>The subtask progression model is not initialized.
  When the subtask progression model reaches its end,
  the value of the task progression is {@code s + extent}
  where {@code s} is the value of the task progression when
  this function is called.
  
  @param model is the model to derivate
  @param extent is the size of the subtask progression that is
  covered in the task progression.
  @return the subtask progression model; or <code>null</code>.