code: def fromone(cls, index, pad=0, autostep=None):
          
          inst = RangeSet(autostep=autostep)
          # support slice object with duck-typing
          try:
              inst.add(index, pad)
          except TypeError:
              if not index.stop:
                  raise ValueError("Invalid range upper limit (%s)" % index.stop)
              inst.add_range(index.start or 0, index.stop, index.step or 1, pad)
          return inst
docs: Class method that returns a new RangeSet of one single item or
          a single range (from integer or slice object).