[help]oracle trigger problem

hi all,
i am a newbie of oracle. i am now doing a assignment and i cant find the way the slove the problem.
the problem is taht:

i am required to create a trigger fired by an update statement. it will decide the tuple can be updated or not based on another table. but i cant find way to get the detail of the updated tuple.

the following is some part of my assigment:

create table movie (
mname VARCHAR2(50) primary key,
year number(4) not null,
did char(1) references director(did),
d_pay number(10,2) constraint check_d_pay check(d_pay<1000000)
);

create table sequel (
original_name VARCHAR2(50) PRIMARY KEY references movie(mname),
sequel_name VARCHAR2(50) NOT NULL
);

when update the d_pay, such as:
update movie set d_pay = 5000 where mname = 'troy';

the trigger are required to check if the movie has sequel or not. if it has sequel, d_pay are not allow to be updated.

can anyone teach me how to reach that purpose??
thx

needs some improvement but this ought to do it?