/ackermann{
/n exch def
/m exch def %PostScript takes arguments in the reverse order as specified in the function definition
m 0 eq{
n 1 add
}if
m 0 gt n 0 eq and
{
m 1 sub 1 ackermann
m 0 gt n 0 gt and{
m 1 sub m n 1 sub ackermann ackermann
}def