Suppose that after submitting a paper for publication, but before hearing anything back from the referee, I discover an error in the paper that needs to be corrected, or an omission that needs to be rectified. What is the best course of action? Should I send a revised version to the editor to forward to the referee, explaining exactly what I have changed? Or wait to hear back from the referee and then include the fixes in my next version along with whatever other changes the referee suggests (again explaining them to the editor)?