anchor.diff:        call thck6d(nthinerr)
       end if
     else
  -    if((idp == 0 .or. ition == 0) .and. .not.do_coll) then ! 4D tracking (not collimat compatible)
  +    if(tr_is4D .and. .not.do_coll) then ! 4D tracking is not collimat compatible
         write(lout,"(a)") ""
         write(lout,"(a)") "TRACKING> Starting Thin 4D Tracking"
         write(lout,"(a)") ""
         trackMode = "Thin 4D"
         call thin4d(nthinerr)
       else
  -      if(do_coll .and. (idp == 0 .or. ition == 0)) then ! Actually 4D, but collimation needs 6D so goto 6D.
  -        write(lout,"(a)") "TRACKING> WARNING Calling 6D tracking due to collimation! Would normally have called thin4d"
  -      endif
  +      if(tr_is4D .and. do_coll) then
  +        write(lout,"(a)") "TRACKING> WARNING Calling 6D tracking due to collimation - would normally have called 4D"
  +      end if
         write(lout,"(a)") ""
         write(lout,"(a)") "TRACKING> Starting Thin 6D Tracking"
         write(lout,"(a)") ""
  
positive.code: ! ================================================================================================ !
  !  F. Schmidt
  !  Created: 1999-02-03
  ! ================================================================================================ !
  subroutine trackDistance
  
  
negative.code: 
    nthinerr = 0
  #ifdef FLUKA
    napxto = 0
  #endif
    if(lbacktracking) call aperture_backTrackingInit
  
    if(ithick == 1) then
      if(tr_is4D) then
        write(lout,"(a)") ""
        write(lout,"(a)") "TRACKING> Starting Thick 4D Tracking"
        write(lout,"(a)") ""
  