laps – Zero-width boxes for annotations, etc

Provides macros extends the \llap/\rlap set in Plain TeX:

• \ulap and /dlap are up and down laps, in the same series as the originals;
• \xlap and \ylap are laps in the horizontal and vertical directions, respectively, and
• \zlap provides a box of zero dimensions in both axes, centred on its midpoint.