 This says that if y is not part of x, there exists something that comprises exactly those parts of y that are disjoint from x—something we may call the difference or relative complement between y and x. It is easily checked that this principle implies (P.5). On the other hand, the diagram in Figure 5 shows that the converse does not hold: there are two parts of y in this diagram that do not overlap x, namely z and w, but there is nothing that consists exactly of such parts, so we have a model of (P.5) in which (P.6) fails.