function _subdiv_char(posX, posY) {
  // NOTE: This only works for grid size 4.
  return GEOCELL_ALPHABET.charAt(
    (posY & 2) << 2 |
    (posX & 2) << 1 |
    (posY & 1) << 1 |
    (posX & 1) << 0);
}
Returns the geocell character in the 4x4 alphabet grid at pos. (x, y).
@return