BPMN21 — This is a style suggestion to make diagrams clearer

This is a style suggestion to make diagrams clearer. The XOR gateway allows two forms of diagram: empty diamond, and a diamond with an X in it. While redundancy is OK in general, the fact that different vendors make different choices. But an X and a cross look very similar. While the X should be allowed, the spec should indicate that it is preferred to use the blank option, with the eye to eventually eliminating the X.