#!/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;
    }
}
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