Return True if all tasks or files are valid.
	Valid tasks have been completed already. Valid files exist on the disk.

def valid(*things):
	'''Return True if all tasks or files are valid.
	Valid tasks have been completed already. Valid files exist on the disk.'''
	for thing in things:
		if type(thing) is str and not os.path.exists(thing):
			return False
		if thing.valid is None:
			return False
	return True