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.
Zipper vertices are marked by arrowheads. (A) advancing zipper, (B) two associated advancing zippers. The total length of the network segments in B decreases during the zippering process.
