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, $u);
    return ($wanted_value, tail($input));
  };