pushouts in REL

To the list: I am actually travelling (in Russia) and I need more or less urgently a reference concerning pullbacks and pushouts in the category of sets and relations - do they always exist etc - I would not adress it to the list if it would be not urgent and I would not have some difficulty with search from here - Best to all - Sergei Soloviev [For admin and other information see: http://www.mta.ca/~cat-dist/ ]