Get current and/or set history cursor position

		:param pos: if value is not None, then current position is set to pos and new value is returned

		:return: int or None (if position have never been changed)

def position(self, pos=None):
		""" Get current and/or set history cursor position

		:param pos: if value is not None, then current position is set to pos and new value is returned

		:return: int or None (if position have never been changed)
		"""
		if pos is not None:
			if pos >= len(self.__history):
				raise IndexError('History position is out of bound')
			self.__history_position = pos
		return self.__history_position