R devel - Oct 2006 - Cox model - (TStart,Tstop] algorith

Hi ALL,
 To fit proportional hazard Cox Model with counting
process style of input data R and S-plus implement
"(TStart,Tstop] algorithm" (agfit3.c) which  uses
"smart subsets" of sorted events time. Are there any
accessible publications where the idea of this
algorithm was represented ?

 Thank you,
    Gregory Kotler