The 2 0 at the start of the APL line above controls the mirroring behavior. The second number can be set to 0 or 1 to choose which side of the image to mirror, while the 2 sets the axis along which to mirror. This will be 1 or 2 for a raster image but this function can mirror any rank of array on any axis.
April was used to teach image filtering in a programming class for middle-schoolers, you can see a summary in this video: https://vimeo.com/504928819
APL is a completely different paradigm.
Every now and then I get the urge to take a deep dive into APL or J but I really need a use case.
Since they are array languages they seem like they might be good for machine learning?