<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def vn(x):
 	""" 	value or none, returns none if x is an empty list 	"""

	if x == []:
		return None
	if isinstance(x, list):
		return '|'.join(x)
	if isinstance(x, datetime):
		return x.isoformat()
	return x