I have been studying OOP coding in preparation for v2.0 mod rewriting. One thing I have not been able to find mention of anywhere on the net is how to substitute a new section of code for an original core file's code (for a modified functionality) using the observer-notifier system.

Generally, when the observer is triggered, it executes the desired code and returns to the triggering point to continue.
This will not work for the kind of thing I (and many others) will need to do, which is to execute a snippet of code instead of a snippet in the core file. If the observer returns to the triggering point, the original code will execute. I need it to return to another point after the original snippet, bypassing it.

When I asked in the w3schools forum, the best answer I got was that the original code would need to be registered as a default event handler and that my code would need to tell the default not to execute.

Can I get any more enlightenment on the specifics of accomplishing this result in the v2.0 structure?