docs: def previous_unwrittable_on_row(view, coords):
      """Return position of the previous (in row) letter that is unwrittable"""
      x, y = coords
      minx = -1
      for offset in range(x - 1, minx, -1):
          letter = view[offset, y]
          if letter not in REWRITABLE_LETTERS:
              return offset
      return None