docs: def next_unwrittable_on_col(view, coords):
      """Return position of the next letter (in column) that is unwrittable"""
      x, y = coords
      maxy = max(view.keys(), key=itemgetter(1))[1]
      for offset in range(y + 1, maxy):
          letter = view[x, offset]
          if letter not in REWRITABLE_LETTERS:
              return offset
      return None