#!/usr/bin/perl --
use strict; use warnings;
print fib(5);
sub fib {
  my( $n, $depth ) = @_;
  $depth ||= 0;
  print "(d $depth)", " " x $depth, " (n $n)\n";
  if ($n < 2) { return $n }
  fib($n-2, $depth + 1 ) + fib($n-1 , $depth + 1 );
}
__END__
(d 0) (n 5)
(d 1)  (n 3)
(d 2)   (n 1)
(d 2)   (n 2)
(d 3)    (n 0)
(d 3)    (n 1)
(d 1)  (n 4)
(d 2)   (n 2)
(d 3)    (n 0)
(d 3)    (n 1)
(d 2)   (n 3)
(d 3)    (n 1)
(d 3)    (n 2)
(d 4)     (n 0)
(d 4)     (n 1)
5