Trying to Creat a Rule

Hello, 

I am trying to create a RULE that updates the size of a class when a student
registers for that class and the register(module) table is updated, and
subtract 1 from the given class size when a student drops a course.
Actually, there is a class table and a module table. The class table's
primary key is id, the module table consists of every module that a student
can register for a given year containing a class id. 

I think I have the INSERT RULE down, but am not sure about the UPDATE RULE: 

CREATE RULE module_update AS 
ON UPDATE TO module.a_q1 
DO 
SET class.size to class.size-1 
WHERE class.id=old.a_q1
AND 
set class.size to class.size+1 
WHERE class.id=new.a_q1; 

I am really not so sure about the 'and' part, but I need to do both updates
on class, using different WHERE criteria. 

TIF, 
Kurt