Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
A repeat v1 … vn and a system of paths overlapping with this repeat. The uppermost path contains the repeat and defines the correct pairing between the corresponding entrance and exit. If this path were not present, the repeat v1 … vn would become a tangle.
