RosettaCodeData/Task/Ackermann-function/Bracmat/ackermann-function-3.bracmat

( AckFormula
=   m n
  .   !arg:(?m,?n)
    & ( !m:0&!n+1
      | !m:1&!n+2
      | !m:2&2*!n+3
      | !m:3&2^(!n+3)+-3
      | !n:0&AckFormula$(!m+-1,1)
      | AckFormula$(!m+-1,AckFormula$(!m,!n+-1))
      )
)