code: func (cps ComparablePrincipalSets) OfMapping(mapping map[int][]int, sets2 ComparablePrincipalSets) comparablePrincipalSetPairs {
  	var res []comparablePrincipalSetPair
  	for i, js := range mapping {
  		for _, j := range js {
  			res = append(res, comparablePrincipalSetPair{
  				contained:  cps[i],
  				containing: sets2[j],
  			})
  		}
  	}
  	return res
  }
docs: // OfMapping returns comparablePrincipalSetPairs comprising only of the indices found in the given keys