docs: def ss_to_xy(s: str):
      """convert spreadsheet coordinates to zero-index xy coordinates.
      return None if input is invalid"""
      result = re.match(r'\$*([A-Z]+)\$*([0-9]+)', s, re.I)
      if result == None:
          return None
      xstring = result.group(1).upper()
      multiplier = 1
      x = 0
      for i in xstring:
          x = x * multiplier + (ord(i) - 64)
          multiplier = multiplier * 26
      x = x - 1
      y = int(result.group(2))-1
      return (x,y)