(alref req!headers "Host")
I can then extend respond to take action for a particular host
(extend respond (req) (begins (alref req!headers "Host") "www.") ...)
(extend respond (req) (begins (alref req!headers "Host") "www.") (prn "HTTP/1.0 302 Moved") (prn "Location: example.com/foo/bar") (prn))
HTTP/1.0 301 Moved Permanently Location: http://www.example.org/foo/bar
-----