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);