Your search returned the following 2 documents:

2. On Kernels, Defaults and Even Graphs
   Yannis Dimopoulos, Vangelis Magirou, and Christos Papadimitriou
   Annals of Mathematics and Artificial Intelligence (MPI-I-93-226), 1993. Note: To appear. Also available as Technical Report MPI-I-93-226
   
1. A Graph Theoretic Approach to Default Logic
   Yannis Dimopoulos and Vangelis Magirou
   Information and Computation 112 (2): 239-256, 1994