Paulo A. S. Veloso,  Sheila R. M. Veloso. 

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.

http://www.lbd.dcc.ufmg.br/colecoes/lsfa/2011/004.pdf

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