code: @Pure
  	@Override
  	public double angle(Vector3D v1) {
  		double vDot = dot(v1) / ( length()*v1.length() );
  		if( vDot < -1.) vDot = -1.;
  		if( vDot >  1.) vDot =  1.;
  		return((Math.acos( vDot )));
  	}
docs: {@inheritDoc}