use warnings;
use strict;

sub A {

    print "\n\nentering, \$_[0] is $_[0]\n";

    print "entering, \$_[1] is $_[1]\n";
    if ($_[1] == 0) {print "1st -- returning 0\n"; return 0  ;}
    if ($_[0] == 0) {print "2nd -- returning 2*\$_[1] ($_[1])\n"; return 2*$_[1] }
    if ($_[1] == 1) {print "3rd -- returning 2\n"; return 2 };
    print "not returning : ";
    # figure A(m-1, A(m, n-1)) using in-place parameter list to save stack
    --$_[1];
    print "\$_[1] is $_[1] ";
    
    $_[1]= &A;

    print "\$_[1] is $_[1] ";
    --$_[0];
    print "\$_[1] is $_[1] ";
    return &A;
}

print A ($ARGV[0], $ARGV[1]);