how to give priority for site with apt

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

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