A question about trigger fucntion syntax

Good morning (at least is is morning East Coast USA time).

I am trying to create a function to validate an attempted record 
insert, and I am having a hard time coming up with syntax that
is acceptable.

Here is the scenario I have a table that has (among other items) employee_key
and work_type_key (both integer FOREIGN KEYS). Then I have another table
that has the following structure:

CREATE  TABLE permitted_work (
    employee_key       integer ,
    work_type_key      integer ,
    permit             boolean DEFAULT FALSE NOT NULL ,
    modtime     timestamptz DEFAULT current_timestamp ,
    FOREIGN KEY (employee_key) references
    employee(employee_key) ,
    FOREIGN KEY (work_type_key) references
    work_type(work_type_key) ,
    CONSTRAINT permit_constraint UNIQUE
            (employee_key , work_type_key)
    );

 What I think I need to do is create a function that is fired on an insert,
 or update to the 1st table that verifies that there is an existing row in
 permitted_work that matches the combination of employee_key AND
 work_type_key AND has the value TRUE in the permit column.

 First does this seem to be a good way to achieve this constraint? If not,
 I am open to suggestions as to other ways to address this  requirement. 

 If it does, could someone give me a little help with th syntax of the
 needed function ??

 Thanks for your time helping me with this.



-- 
"They that would give up essential liberty for temporary safety deserve
neither liberty nor safety."
                        -- Benjamin Franklin