docs: def to_set(self):
          '''Convert the source sequence to a set.
  
          Note: This method uses immediate execution.
  
          Raises:
              ValueError: If duplicate keys are in the projected source sequence.
              ValueError: If the Queryable is closed().
          '''
          if self.closed():
              raise ValueError("Attempt to call to_set() on a closed Queryable.")
  
          if isinstance(self._iterable, set):
              return self._iterable
          s = set()
          for item in self:
              if item in s:
                  raise ValueError("Duplicate item value {0} in sequence "
                      "during to_set()".format(repr(item)))
              s.add(item)
          # Ideally we would close here
          #self.close()
          return s