Apropos of nothing, here’s a Grace version of Ackermann’s Function inspired by Sam Tobin-Hochstadt’s Racket example:

method ack (m:Number, n:Number) -> Number { print "ack {m} {n}" if (m ≤ 0) then { n + 1 } elseif {n ≤ 0} then { ack((m-1), 1) } else { ack(m-1, ack(m, n-1)) } }

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method ack (m:Number, n:Number) -> Number { print "ack {m} {n}" if (m ≤ 0) then { n + 1 }     elseif {n ≤ 0} then { ack((m-1), 1) }     else { ack(m-1, ack(m, n-1)) } }

One interesting thing to note is that the condition after elseif is in {braces}, while the conditions after if is in (parens). This makes logical sense — because the first condition is always evaluated — but is of course inconsistent with every other condition.  (Apologies for the fixed-width font.)