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  use Stream qw/node drop list_to_stream/;

  my @tokens = (
...
  while (my $node = drop($stream)) {
    print Dumper($node, $stream);
  }

  $node   = [ 'OP', '+' ];
  $stream = [               # AoA
    [ 'VAR', 'x' ],
...

  $node   = 'VAL';
  $stream = 3;              # scalar

  my $parser = parser {
    my $input = shift;
    return unless defined $input;
...
    my $wanted_value = $value->($next, $u);
    return ($wanted_value, tail($input));
  };

  my $parser = parser {
    my $input = shift;
    return unless defined $input;
...
    }
    return ($wanted_value, $tail);
  };

  my $parser = concatenate(
    lookfor('OP'),
    lookfor('VAR'),
...
    'concatenate should return the parsed values';
  is_deeply $remainder, [[ VAL => 3 ]], # AoA, not just an aref
    '... and the rest of the stream';

  $parser = concatenate(
    lookfor('OP'),
    lookfor('VAR'),
    lookfor('VAL'),
  );