prog def invt
version 2.1
    mac def _x=1
    mac def _x0=0
    while abs(%_x-%_x0)>1e-3 {
         mac def _x0=%_x
         mac def _f=tprob(%_2,%_x0)-%_1
         mac def _fp=(tprob(%_2,%_x0+.01)-%_1-%_f)/.01
         mac def _x=%_x-%_f/%_fp
}
di "t = " %_x
end