echo nested with a virtual

hello one and all,

using only apache, and its server-side includes, i would like to set a
variable, then use that variable in another include, usually a variable.

so like

<!--#set var="std_path" value="s_path_to_file"-->
<!--#include virtual="<!--#echo var="std_path"-->/s_summary.txt"-->

but apache won't process the nested echo. how can i do this?

lucas