Tunneling application X app over ssh from client to server

[Micha Feigin <michf@post.tau.ac.il>]

due to firewall limitations I have a computer that I can only connect
to indirectly using two ssh tunnels that I haven't managed to tunnel X
over yet.
I can however at that point open a direct connection from the remote
computer to the local one, where I have complete control over both
sides of the connection.
I want to open an X application on the remote machine which will show up
on the local one, where the X data is tunneled over ssh where ssh is
started from the remote machine to the local one.
Hoping that I didn't mess the explenation up too much, any idea how to
do this (what do I use for the display value/do I need to change
anything in sshd?)

A simple diagram to try and clear things

     shh ----------------------> sshd
     app ----------------------> display
[ remote box]                [ local box ]

(there is a connection from the local to the remote box not shown here
since I can't tunnel the X connection over it)