def add_ext(name, ext, seq=-1):
	"""add_ext constructs file names with a name, sequence number, and 
	extension.

	Args:
		name (string) - the filename
		ext (string) - the file extension
		seq (int) - the number in a sequence with other files 
					(Default -1) means that it is a standalone 
					file
	"""

	#add period if not
	if '.' not in ext[0]:
		ext = '.' + ext 

	if seq != -1:
		ext = str(seq) + ext 
	
	return name + ext