code: func (ps PrincipalSet) ContainingOnly(f func(*msp.MSPPrincipal) bool) bool {
  	for _, principal := range ps {
  		if !f(principal) {
  			return false
  		}
  	}
  	return true
  }
docs: // ContainingOnly returns whether the given PrincipalSet contains only Principals
  // that satisfy the given predicate