code: public void grantPermission(Grantee grantee, Permission permission) {
          getGrantsAsList().add(new Grant(grantee, permission));
      }
docs: Adds a grantee to the access control list (ACL) with the given permission.
  If this access control list already
  contains the grantee (i.e. the same grantee object) the permission for the
  grantee will be updated.
  
  @param grantee
  The grantee to whom the permission will apply.
  @param permission
  The permission to apply to the grantee.