my $parser = parser {
    my $input = shift;
    return unless defined $input;
    my $next = head($input);
    for my $i (0 .. $#$wanted) {
      next unless defined $wanted->[$i];
      return unless $wanted->[$i] eq $next->[$i];
    }
    my $wanted_value = $value->($next, $u);
    my $tail = tail($input);

    # is this the last token from the stream?
    if ($tail && 'ARRAY' eq ref $tail && 'ARRAY' ne ref $tail->[0]) {
        $tail = [ $tail ];
    }
    return ($wanted_value, $tail);
  };