On Graph Refutation for Relational Inclusions
  We introduce a graphical refutation calculus for relational inclusions: it reduces establishing a relational inclusion to establishing that a graph constructed from it has empty extension. This sound and complete calculus is conceptually simpler and easier to use than the usual ones. 
Paulo A. S. Veloso (COPPE-UFRJ, Brazil), Sheila R. M. Veloso   (FEN-UERJ, Brazil)
cs.LO