SPI_modifytuple - why i cant use it in trigers fired after

fragment from PostgreSQL 7.1 Documentation:
"Trigger functions return HeapTuple to the calling Executor. This is
ignored for triggers fired after an INSERT, DELETE or UPDATE operation but
it allows BEFORE triggers to: ......"

can somebody tell my why ? is it bug or what ?
sometimes i have to use it AFTER insert, update or delete. what can i do
in this situation.

thank you in advance


--
Marek Bartnikowski <mailto:marek@virt.pl>
PostgreSQL DBA
C, PHP Prog.