z

layout height, defaulting to zero rather than to not-a-number. A guide path with no layout at all is a flat one at ground level, which is a meaningful position; a height of not-a-number would be a third way of saying "unspecified" that every reader would have to handle, and would poison any renderer that added it to a camera. The two planar coordinates keep not-a-number because there it distinguishes "no layout" from "the origin", and a reader testing x.isNaN() for that still gets the right answer.