public static RectD UnionWith(this RectD r1, RectD r2) => RectD_union(r1, r2);
public static bool RectD_intersect(RectD r1, RectD r2) { return(r1.IntersectionWith(r2) != null); }
public static RectD IntersectionWith(this RectD r1, RectD r2) => RectD_intersection(r1, r2);