public Arc(VoronoiCell cell, Point p, Arc prev) : this(cell, p)
 {
     this.prev = prev;
 }
 public Arc(VoronoiCell cell, Point p, Arc prev, Arc next) : this(cell, p, prev)
 {
     this.next = next;
 }
 public Arc(VoronoiCell cell, Point p)
 {
     this.p    = p;
     this.cell = cell;
 }