Hi MiKaël,
I have done a bit of what I wanted. The tasks have now the attribute
dependson, which is a list of integers. It appears with the char O in
'ls'. This is not completely nice for me; it has two problems:

a) the priority is not changed. The computation of priorities is done on
a task basis, and tasks do not know which ones depends on them. I can
add a new property for tasks, 'dependedby' or something like this, but
it does not look like a good solution.

b) if you change the todo.txt file by hand, you can completely corrupt
the dependencies, since the only info I have for dependencies is their
number.

Maybe both problems could be solved by adding, at the end of todo.txt, a
list of dependencies, not by number, but by title. Something like

------------------------ todo.txt ---------------------------
Talk to boss U:1 I:3 T:1H S:2009-03-05
End nice project p:nice U:4 I:3 T:1H S:2009-03-05
Begin odd project p:odd U:2 I:2 T:5M S:2009-03-05

Dependencies:
Begin odd project
End nice project
Talk to boss
End nice project
Talk to boss
------------------------ todo.txt ---------------------------

I'm sending you the yagtd.py and gtd.py that I have so far. They have
marks as
# Matias beg
new stuff
# Matias end
which I put for finding new additions easily. Of course, I will
delete those marks when they are not needed anymore.

Best regards,
Matías