Hartmut Ehrig Reiko Heckel Mercè Llabrés Fernando Orejas Julia Padberg Grzegorz Rozenberg Double-Pullback Graph Transitions: A Rule-Based Framework with Incomplete Information. 85-102 1998 conf/tagt/1998 TAGT db/conf/tagt/tagt1998.html#EhrigHLOPR98