About hacking postgresql

Hellow,
 
I want to be more precis
I want to change the syntax as follows
Start Transaction (priority)
 
I have 03 transactions
I want to give to every transaction à priority
 
Example
Transaction 1
Start transaction (03)
………….
Commit
 
Transaction 2
Start transaction (04)
………
Commit
 
Transaction 1
Start transaction (01)
…………..
Commit
 
(the transaction with the higth priority must execute first)
 
How to express this in gram.y? Is change in gram.y sufficient?
 
Thanks a lot