code: def remove(self, element):
          
          if element not in self._counts:
              raise KeyError(element)
          elif self._counts[element] == 1:
              newc = self._counts.remove(element)
          else:
              newc = self._counts.set(element, self._counts[element] - 1)
          return PBag(newc)
docs: Remove an element from the bag.
  
          >>> s = pbag([1, 1, 2])
          >>> s2 = s.remove(1)
          >>> s3 = s.remove(2)
          >>> s2
          pbag([1, 2])
          >>> s3
          pbag([1, 1])