code: private static void printCodePointAsSurrogatePairHexDigits(Appendable out, int c)
          throws IOException
      {
          for (final char unit : Character.toChars(c)) {
              printCodePointAsFourHexDigits(out, unit);
          }
      }
docs: Generates a surrogate pair as two four-digit hex escape sequences,
  {@code "\}{@code u}<i>{@code HHHH}</i>{@code \}{@code u}<i>{@code HHHH}</i>{@code "},
  using lower-case for alphabetics.
  This for necessary for JSON when the code point is outside the BMP.