one of the documented procedures in this installation of the ACS
ad_return_top_of_page   first_part_of_page   { content_type "text/html" }
What it does:
Returns HTTP headers plus the top of the user-ivisible page. Saves a TCP packet (and therefore some overhead) compared to using ReturnHeaders and an ns_write.
Defined in: /web/philip/packages/acs-core/utilities-procs.tcl

Source code:

    set all_the_headers "HTTP/1.0 200 OK
MIME-Version: 1.0
Content-Type: $content_type\n"
     util_WriteWithExtraOutputHeaders $all_the_headers $first_part_of_page