I'm sorry, it has a lot of code and it took me a while to find this documentation.
I found it in "examples" and "SQLApp"
-> book.WhenLeftDouble = THISBACK(EditBook);
if you want you can close this topic.
appreciate
Not all examples are updated to reflect latest improvements in U++. Here it is worth mentioning that lately you can completely specify the action using lambda:
book.WhenLeftDouble << [=] { some code describing what should happen };