# File lib/ec2/right_ec2_ebs.rb, line 269
    def modify_snapshot_attribute(snapshot_id, attribute='createVolumePermission', operation_type='add', vars = {})
      params =  {'SnapshotId'    => snapshot_id,
                 'Attribute'     => attribute,
                 'OperationType' => operation_type}
      params.update(amazonize_list('UserId',    Array(vars[:user_id])))    if vars[:user_id]
      params.update(amazonize_list('UserGroup', Array(vars[:user_group]))) if vars[:user_group]
      link = generate_request("ModifySnapshotAttribute", params)
      request_info(link, RightBoolResponseParser.new(:logger => @logger))
    rescue Exception
      on_exception
    end