Source PostScript (exch.pps)

/exchp {
   4 -1 roll
   4 -1 roll
} def
/exchc {
   6 -1 roll
   6 -1 roll
   6 -1 roll
} def
/exchd {
   4 {8 -1 roll} repeat
} def
/exchp3d {
   6 -1 roll
   6 -1 roll
   6 -1 roll
} def