import glob


def get_possible_sites(user):
	"""Returns a set of sites available for the specified user. Each site 
	is represented by an image in the 'images' directory. """
	files=()
	if user.lower().endswith("ur"):
		#ASSERT: the username indicates unrelated condition
		files=(file.replace('images/', '') for file in glob.glob('images/ur_*'))
	elif user.lower().endswith("r"):
		#ASSERT: the username indicates related condition
		files=(file.replace('images/', '') for file in glob.glob('images/r_*'))
	return set(files)