sub classify_line { # return (indent, type, modified_line, info)
      local $_ = shift;

      s/^(\s*)$//          && return (length($1), $BLANK, $_, undef);
      s/^(\s*\*\s+)//      && return (length($1), $BULLET, $_, undef);
      s/^(\s*(\d+)\.\s+)// && return (length($1), $NUMBER, $_, $2);
      s/^(\s*)//           && return (length($1), $PLAIN, $_, undef);
      die "classify_line:\n\t" . $_ . "\n";
  }