I personally found this example a bit confusing on first (and third) read... Could it be made clearer that `origin` is a location in *image* coordinates?  Perhaps add the line: `origin = galsim.PositionD(-45., 13.) # Position in image coordinates to use as a new origin` or similar?   I think the form below might be most logical:
```
                origin = galsim.PositionD(-45., 13.) # Position in image coordinates to use as a new origin
                shifted = wcs.setOrigin(origin)
                world_pos1 = wcs.toWorld(PositionD(0,0))
                world_pos2 = shifted.toWorld(origin)
                # world_pos1 should be equal to world_pos2
```
Then again, it's just me!