docs: def common_start(*args):
      """ returns the longest common substring from the beginning of sa and sb """
      def _iter():
          for s in zip(*args):
              if len(set(s)) < len(args):
                  yield s[0]
              else:
                  return
  
      out = "".join(_iter()).strip()
      result = [s for s in args if not s.startswith(out)]
      result.insert(0, out)
      return ', '.join(result)