code: def union(self, a, b):
          
          s1, s2 = self.find(a), self.find(b)
          if s1 != s2:
              r1, r2  = self._rank[s1], self._rank[s2]
              if r2 > r1:
                  r1, r2 = r2, r1
                  s1, s2 = s2, s1
              if r1 == r2:
                  self._rank[s1] += 1
  
              self._leader[s2] = s1
              self._size[s1]  += self._size[s2]
              self.nClusters  -= 1
docs: Merges the set that contains ``a`` with the set that contains ``b``.
  
          Parameters
          ----------
          a, b : objects
              Two objects whose sets are to be merged.