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