This is a standard LensDistortion node but with added soft mask functionality. It uses the expressions u*width-x and v*height-y to convert from UV space to pixel coordinates, allowing you to soft mask the distortion without any doubling up of the image.
Unfortunately I haven't figured out yet how to undistort this as inverting the stmap isn't so straightforward, but this node still works as a quick and light way to add a creative lens distort to specific areas of your shot.