code: func (p *Policy) GrantRole(role string, principals ...string) {
  	if len(principals) == 0 {
  		return
  	}
  	if p.Bindings == nil {
  		p.Bindings = make(PolicyBindings, 1)
  	}
  	members := p.Bindings[role]
  	if members == nil {
  		members = make(membersSet, len(principals))
  		p.Bindings[role] = members
  	}
  	for _, principal := range principals {
  		members[principal] = struct{}{}
  	}
  }
docs: // GrantRole grants a role to the given set of principals.