From Willy: Define a standard for helper parts

[Milan Vančura]

Why forbid that, Isn't it up to the 'style' of the (e.g.) arrow collection to include edges or not.

This is an interesting point. What about this rule instead: "Edges must be always in the edge color. If the author's idea is to have them in the main color, they (edges) should be omit instead."