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