code: function Circle3D( x , y , z , nx , ny , nz , r ) {
  	this.center = new Vector3D( x , y , z ) ;
  	this.r = + r ;
  	this.planeNormal = new Vector3D( nx , ny , nz ).normalizeCheck() ;
  }
docs: /*
  A Circle3D is the intersection of a Plane3D and a Sphere3D.
  The center of the sphere is on the plane.
  
  x,y,z and r are like in Sphere3D.
  planeNormal is the normal vector of the Plane3D.