\ Ackermann's function in FORTH/6000
\ GLM, October 2026

: ACK ( m n -- r )
  OVER 0= IF NIP 1+ EXIT THEN
  DUP 0= IF DROP 1- 1 RECURSE EXIT THEN
  2DUP 1- RECURSE >R DROP 1- R> RECURSE ;

." ack(2,3) = " 2 3 ACK . CR
." ack(3,3) = " 3 3 ACK . CR
." ack(3,5) = " 3 5 ACK . CR
." ack(3,6) = " 3 6 ACK . CR

." ack(3,6), one run: " CLOCK 3 6 ACK DROP CLOCK SWAP - U. ." us" CR