docs: def add(self, element, multiplicity=1):
          """Adds an element to the multiset.
  
          >>> ms = Multiset()
          >>> ms.add('a')
          >>> sorted(ms)
          ['a']
  
          An optional multiplicity can be specified to define how many of the element are added:
  
          >>> ms.add('b', 2)
          >>> sorted(ms)
          ['a', 'b', 'b']
  
          This extends the :meth:`MutableSet.add` signature to allow specifying the multiplicity.
  
          Args:
              element:
                  The element to add to the multiset.
              multiplicity:
                  The multiplicity i.e. count of elements to add.
          """
          if multiplicity < 1:
              raise ValueError("Multiplicity must be positive")
          self._elements[element] += multiplicity
          self._total += multiplicity