Merge pull request #651 from bmad-sim/devel/57

Fix potential round-off error problem.

bmad/low_level/track1_runge_kutta.f90

@@ -92,8 +92,8 @@ subroutine track1_runge_kutta (orbit, ele, param, err_flag, track, mat6, make_ma
 ! Track.
 ! Note that if ele is a slave, ele%field_calc = refer_to_lords$ and no error message is printed. 
 
-if ((ele%key == lcavity$ .or. ele%key == rfcavity$) .and. &
-                ele%field_calc == bmad_standard$ .and. ele%value(l$) < ele%value(l_active$)) then
+if ((ele%key == lcavity$ .or. ele%key == rfcavity$) .and. ele%field_calc == bmad_standard$ .and. &
+                ele%value(l$) < ele%value(l_active$) - 0.1*bmad_com%significant_length) then
   call out_io (s_error$, r_name, 'RUNGE-KUTTA TRACKING THROUGH A CAVITY: ' // ele%name, &
                           'WILL NOT BE ACCURATE SINCE THE LENGTH IS LESS THAN THE HARD EDGE MODEL LENGTH.')
 endif