how to give priority for site with apt

["abdelkader belahcene" <abelahcene@gmail.com>]

hi,
I am looking for possibility to make priority for repository.
In fact I have a local repository, when I want to install a package I
want apt to check first in the local to install it from there if it
exists.
for example
here is my sources.list:
deb file:/opt/ R4.1_2/
deb file:/opt/R4.1_1  etch main contrib
deb http://ftp.fr.debian.org/debian lenny main contrib non-free

If a software (say lyx-1.5.1), exists  in /opt/R4.1_2/ I want to
install it from here and not from ftp.fr.debian.org/debian.  where the
version is the same of course.

I haven't found info about priority in the apt doc????

thanks for help
bela