The "AuthNullCallback" proc implements a sort of auto-login feature.
If you login with the same username and password twice, it remembers
that (until the server is restarted).
<p>
[if {[info exist env(REMOTE_USER)]} {
    set _ "You logged in as $env(REMOTE_USER)"
} elseif {[info exist env(HTTP_AUTHORIZATION)]} {
    set _ "HTTP_AUTHORIZATION $env(HTTP_AUTHORIZATION)"
} else {
    set _ Hello World
}]
<p>
[clock format [clock seconds]]