Fix bug in Abs generation, the positions weren't recorded.

Commit `b0dc41f3a7b59c3a3eae1942f3ab879edefbfab4` broke the `position` command.  The reason for the breakage is that `isPositionCat` returns `False` with `TokenCat`s.

This fixes it, but I don't know what the intendent behaviour of `isPositionCat` is -- I looked at the source code very briefly.