code: @Override
      public DBSubnetGroup modifyDBSubnetGroup(ModifyDBSubnetGroupRequest request) {
          request = beforeClientExecution(request);
          return executeModifyDBSubnetGroup(request);
      }
docs: <p>
  Modifies an existing DB subnet group. DB subnet groups must contain at least one subnet in at least two AZs in
  the AWS Region.
  </p>
  
  @param modifyDBSubnetGroupRequest
  @return Result of the ModifyDBSubnetGroup operation returned by the service.
  @throws DBSubnetGroupNotFoundException
  <i>DBSubnetGroupName</i> does not refer to an existing DB subnet group.
  @throws DBSubnetQuotaExceededException
  Request would result in user exceeding the allowed number of subnets in a DB subnet groups.
  @throws SubnetAlreadyInUseException
  The DB subnet is already in use in the Availability Zone.
  @throws DBSubnetGroupDoesNotCoverEnoughAZsException
  Subnets in the DB subnet group should cover at least two Availability Zones unless there is only one
  Availability Zone.
  @throws InvalidSubnetException
  The requested subnet is invalid, or multiple subnets were requested that are not all in a common VPC.
  @sample AmazonNeptune.ModifyDBSubnetGroup
  @see <a href="http://docs.aws.amazon.com/goto/WebAPI/neptune-2014-10-31/ModifyDBSubnetGroup" target="_top">AWS
  API Documentation</a>