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.
In the version of the article initially published, all instances of “HAT” appeared as “XAT”. This has now been corrected in the HTML and PDF versions of the article.
