code: def add_process(self, command=None, vsplit=False, start_directory=None):
          
          assert command is None or isinstance(command, six.text_type)
          assert start_directory is None or isinstance(start_directory, six.text_type)
  
          window = self.arrangement.get_active_window()
  
          pane = self._create_pane(window, command, start_directory=start_directory)
          window.add_pane(pane, vsplit=vsplit)
          pane.focus()
          self.invalidate()
docs: Add a new process to the current window. (vsplit/hsplit).