请问oracle 里 foreign key的问题

CREATE OR REPLACE TYPE Student_type AS OBJECT(
  SSN varchar(30),
  name varchar(30),
  DOB DATE,
  Gender CHAR(1),
  age integer,
  address Address_type, 
)NOT FINAL;
/


CREATE OR REPLACE TYPE Master_type UNDER Student_type(  
)NOT FINAL;
/

CREATE OR REPLACE TYPE PHD_type UNDER Student_type(
  Advisor varchar(30),
  Program varchar(10)
);
/

CREATE TABLE students of Student_type(
  Primary key(SSN)
);

CREATE TABLE MasterAssigned(
  stuId varchar(30),
  taskId varchar(20),
  schedule TimeSchedule,
  Primary key(stuID,taskId,schedule),
  Foreign key(stuId) references Students(SSN) 
  Students IS OF (ONLY Master_type),
  Foreign key(taskId) references Tasks(TaskID) TREAT Tasks.Object_Value AS PictureTask
);

MasterAssigned 里stuId 如果只指master type的话， 这个foreign key 语句该怎么写呢？
谢谢大家了 

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CREATE TABLE MasterAssigned( 
  stuId varchar(30), 
  taskId varchar(20), 
  schedule TimeSchedule, 
  Primary key(stuID,taskId,schedule), 
  constraint fk_name foreign key (stuId) r