Forward transformation is from pixels --> coordinate_system, the inverse transformation is to pixels.
After listening to arguments both ways I am starting to think that intermediate coordinate systems must be added. `astropy.wcs` provides similar functionality, although through methods like `pix2foc` and `sky2foc` and I find myself using these quite often. But I have to think how to implement this. I like AST's way a bit better because it's more flexible.