%%% File: Laps.TeX
\catcode`\@=11   % Let's pretend @ is a letter 
%         Vertical `laps'; cf. \llap and \rlap
\long\def\ulap#1{\vbox to \z@{\vss#1}}
\long\def\dlap#1{\vbox to \z@{#1\vss}}
     
%         And centered horizontal and vertical `laps'
\def\xlap#1{\hbox to \z@{\hss#1\hss}}
\long\def\ylap#1{\vbox to \z@{\vss#1\vss}}
     
%         And a `lap' centered on its midpoint
\long\def\zlap#1{\ylap{\xlap{#1}}}

\catcode`\@=12   % That's enough pretending for one day.