#!/usr/bin/perl
use strict;
no warnings ;

my $tree1 = [ [ [ 'a', 'b', ], 'c'],'d' ];
my $tree2 = [ ['a','b'],[ 'c','d' ] ];

my $ti1 = get_tree_iterator($tree1);
my $ti2 = get_tree_iterator($tree2);

my $cnt_mismatches=0;
while (1) {
    my ($L, $R) = ($ti1->(), $ti2->());
    print "L=$L, R=$R\n";
    die "--- not equal!" unless  ($R eq $L) or (!$L and !$R);
    last if !defined $L;
}
print $cnt_mismatches ? "$cnt_mismatches mismatches" : "TREES MATCH!", "\n";

use Data::Dump 'dd';

sub get_tree_iterator {
    my @stack = (shift);
    return sub {
      print "---\n";
      dd \@stack;
      while (@stack and ref $stack[0] eq 'ARRAY') { 
	unshift @stack, @{shift @stack};
	dd \@stack
      }
      return shift @stack;
    }
}

##</code><code>##

---
[[[["a", "b"], "c"], "d"]]
[[["a", "b"], "c"], "d"]
[["a", "b"], "c", "d"]
["a", "b", "c", "d"]
---
[[["a", "b"], ["c", "d"]]]
[["a", "b"], ["c", "d"]]
["a", "b", ["c", "d"]]
L=a, R=a
---
["b", "c", "d"]
---
["b", ["c", "d"]]
L=b, R=b
---
["c", "d"]
---
[["c", "d"]]
["c", "d"]
L=c, R=c
---
["d"]
---
["d"]
L=d, R=d
---
[]
---
[]
L=, R=