implement union-find for cancel functions
implement union-find for cancel functions

Ignore-this: 16f124923bebaad3099920c28717420f

darcs-hash:20110712143902-c41ad-5aa86a80728bdc454e2d0b71525a6ab464db5447

gfxmonk/lwt,mfp/lwt,mfp/lwt,rgrinberg/lwt,rgrinberg/lwt,janestreet/lwt-async,rneswold/lwt,c-cube/lwt,ocsigen/lwt,c-cube/lwt,ocsigen/lwt,fdopen/lwt,janestreet/lwt-async,ocsigen/lwt,c-cube/lwt,rgrinberg/lwt,gfxmonk/lwt,mirage/lwt,rneswold/lwt,rneswold/lwt,rgrinberg/lwt,fdopen/lwt,mirage/lwt