Luís Cruz-Filipe Amílcar Sernadas Cristina Sernadas Heterogeneous Fibring of Deductive Systems Via Abstract Proof Systems. 121-153 2008 16 Logic Journal of the IGPL 2 http://dx.doi.org/10.1093/jigpal/jzm057 db/journals/igpl/igpl16.html#Cruz-FilipeSS08