Plane: "normal vector pointing towards the origin" is confusing

[ghost]

As far as I understand, the normal vector of the plane can point in the reverse order than towards the origin.

In graphene_plane_init(), 'constant' is the negative distance. So should the value of 'constant' be positive or negative? Are both positive and negative allowed?

What does graphene_plane_normalize() do? I think it should be documented in the class description. I guess a normalized plane is when the normal vector really points to the origin.

For graphene_plane_negate(), "Negates the normal vector and constant" -> if both values are negated, the result is the same plane, no? Or I've misunderstood something, and it's related to the above comments.