def abs_file_name(dir_, file_name):
"""If 'file_name' starts with '/', returns a copy of 'file_name'.
Otherwise, returns an absolute path to 'file_name' considering it relative
def abs_file_name(dir_, file_name):
"""If 'file_name' starts with '/', returns a copy of 'file_name'.
Otherwise, returns an absolute path to 'file_name' considering it relative