code: func (psSets PrincipalSets) ContainingOnly(f func(*msp.MSPPrincipal) bool) PrincipalSets {
  	var res PrincipalSets
  	for _, set := range psSets {
  		if !set.ContainingOnly(f) {
  			continue
  		}
  		res = append(res, set)
  	}
  	return res
  }
docs: // ContainingOnly returns PrincipalSets that contain only principals of the given predicate