docs: public static IntegralKernel kernelDerivY( int r , IntegralKernel ret ) {
  		if( ret == null )
  			ret = new IntegralKernel(2);
  
  		ret.blocks[0].set(-r-1,-r-1,r,-1);
  		ret.blocks[1].set(-r-1,0,r,r);
  		ret.scales[0] = -1;
  		ret.scales[1] = 1;
  
  		return ret;
  	}