The ZX-calculus is incomplete for quantum mechanics
  We prove that the ZX-calculus is incomplete for quantum mechanics. We suggest the addition of a new 'color-swap' rule, of which currently no analytical formulation is known and which we suspect may be necessary, but not sufficient to make the ZX-calculus complete. 
Christian Schr\"oder de Witt, Vladimir Zamdzhiev (University of   Oxford)
cs.LO math.CT quant-ph