sub twig_iterator
{
  my $file = shift;
  my $cb   = Coro::rouse_cb;
  my $twig = XML::Twig->new(
    twig_handlers => {
      elem => sub { $cb->(elem => @_) }
      otherelem => sub { $cb->(otherelem => @_) }
    },
  );
  my $done;

  # $cb->() rouses with no parameters.
  async { shift->parse(); $cb->() } $twig;

  sub {
    Coro::rouse_wait($cb); # will return the parameters received by $cb above
  }
}

my $itA = twig_iterator($fileA);
my $itB = twig_iterator($fileB);

while (1)
{
  # if array has no items, it's done parsing, otherwise:
  # [0] == elem name (hardcoded in above)
  # [1..$#array] == items passed in by XML::Twig to the callback
  my @A = $itA->();
  my @B = $itB->();

  # compare?
}