How do I write a program for Ackermann function in Stata?

[Nobuaki Michihata]

I wrote a program for the Ackermann function in Stata.
The link below described how to do this using mata, but
without using mata it would look like this, what am I doing wrong?

https://rosettacode.org/wiki/Ackermann_function

program ackermann, rclass
gettoken a 0 : 0, parse(" ")
gettoken b 0 : 0, parse(" ")

confirm integer number `a'
confirm integer number `b'

if `a' == 0 {
di `b' + 1
return scalar N=`b' + 1
}
else if `a' > 0 & `b'==0 {
local j=`a'-1
ackermann `j' 1
}
else if `a' > 0 & `b' > 0 {
local k=`b'-1
qui ackermann `a' `k'
local c=`r(N)'
local h=`a'-1
ackermann `h' `c'
}
end

ackermann 0 1
return list
ackermann 1 0
return list
ackermann 1 1
return list