code: func computeContainedInMapping(s1, s2 []ComparablePrincipalSet) intMapping {
  	mapping := make(map[int][]int)
  	for i, ps1 := range s1 {
  		for j, ps2 := range s2 {
  			if !ps1.IsContainedIn(ps2) {
  				continue
  			}
  			mapping[i] = append(mapping[i], j)
  		}
  	}
  	return mapping
  }
docs: // computeContainedInMapping returns a mapping from the indices in the first ComparablePrincipalSets
  // to the indices in the second ComparablePrincipalSets that the corresponding ComparablePrincipalSets in
  // the first ComparablePrincipalSets are contained in the second ComparablePrincipalSets given.