[lltx] luatexbase revisited

[Arno Trautmann]

Hi all,

some time ago I made a feature request for the luatexbase-mcb package. I 
wanted the user to be able to check wether a function is registered in a 
callback. [1]
At that time, I only got answers by Patrick who told me that I didn't 
need such functionality, and in fact it didn't matter with what I did 
then. But now I have a more complex function that I want to register. 
What I want:
The user has two macros \startsomething and \stopsomething. These macros 
set and remove LuaTeX attributes, respectively. In each paragraph where 
these macros are used, a function in the pre_linebreak_filter should do 
something, but not in other paragraphs. So \startsomething should add 
the function to the callback, but only if it is not already registered. 
The function will then remove itself from the callback when executed. 
luatexbase already checks this, and instead of throwing an error, a 
function like provide_callback(name,func,description,priority) should 
just be quiet.

Sorry, but I still don't see what harm such a function could do and why 
it is not provided already.

Any deeper reasons or am I just the only one misusing this feature?

cheers
Arno

[1] http://tug.org/pipermail/lualatex-dev/2011-July/001266.html