The commands \clap, \llap and \rlap are useful "horizontal" analogues of \smash: they reduce the horizontal width of a box to zero, aligning the box with either the middle, left or right of the typeset contents. Unfortunately they don't work in math mode.
Can someone suggest a math mode macro that has the effect of
\clapand friends?
The mathtools package contains (among other useful things) the implementations of \mathclap and relatives from Andrew's answer.
I don't know if they are implemented in a specific package (search CTAN), but there's an article in TUGBoat, (TUGboat 22 (2001), 350–352.) that gives an implementation. Here's the article on the author's website:
http://math.arizona.edu/~aprl/publications/mathclap/
I suspect that anyone putting this in a package would have used the same names, or at least referred to this article, so searching on CTAN for the command names used there would be a reasonable thing to do.
(Breadcrumb trail: I first learnt of this via this blog post about these commands in iTeX.)
(I'm feeling generous: I just tried the obvious searches on CTAN and nothing came up; so based on that, you should copy the definitions from that article.)
FWIW, in ConTeXt \rlap etc do the right thing in math mode as well. They are defined as
\unexpanded\def\rlap{\mathortext\domathrlap\dorlap}
\unexpanded\def\llap{\mathortext\domathllap\dollap}
\unexpanded\def\clap{\mathortext\domathclap\doclap}
where \domathrlap and \dorlap are roughly equivalent to LaTeX's \mathrlap and \rlap.
