code: def _stringify_na_values(na_values):
      
      result = []
      for x in na_values:
          result.append(str(x))
          result.append(x)
          try:
              v = float(x)
  
              # we are like 999 here
              if v == int(v):
                  v = int(v)
                  result.append("{value}.0".format(value=v))
                  result.append(str(v))
  
              result.append(v)
          except (TypeError, ValueError, OverflowError):
              pass
          try:
              result.append(int(x))
          except (TypeError, ValueError, OverflowError):
              pass
      return set(result)
docs: return a stringified and numeric for these values