heap_modifytuple() - help needed

Can anybodt give me a few examples on how to use heap_modifytuple()
efficiently when I do NOT want to modify an attribute through a postgres
function?

All the examples given in the source seem to use some return of a postgres
function (like current user etc). I just want to insert strings into a tuple
cought in a trigger function. I tried to find some useful information
regarding the "Datum" type, but I failed to understand how to use it here for
my purposes.

Any help appreciated - in exchange I volunteer to write proper documentation
for heap_modifytuple() once I understood how to work it properly.

Regards
Horst