The command smash is used to typeset a subformula and
give it an effective height and depth of zero, which is sometimes
useful in adjusting the subformula's position with respect to adjacent
symbols. With the amsmath package smash has optional arguments
|t| and |b|, because occasionally it is advantageous to be able to
``smash'' only the top or only the bottom of something while retaining
the natural depth or height. For example, when adjacent radical
symbols are unevenly sized or positioned because of differences in the
height and depth of their contents, smash can be employed to make
them more consistent. Compare
and 
,
where the latter was produced by
SPM_quot
".