/==!/==atoi=@@@-@-----#
   |   |                          Ackermann function
   |   |       /=========\!==\!====\  recursion:
$,@/>,@/==ack=!\?\<+#    |   |     |   A(0,j) -> j+1
 j   i           \<?\+>-@/#  |     |   A(i,0) -> A(i-1,1)
                    \@\>@\->@/@\<-@/#  A(i,j) -> A(i-1,A(i,j-1))
                      |  |     |
            #      #  |  |     |             /+<<<-\
            /-<<+>>\!=/  \=====|==!/========?\>>>=?/<<#
            ?      ?           |   \<<<+>+>>-/
            \>>+<<-/!==========/
            #      #