TeX/abovewithdelims
Synopsis
\abovewithdelims[delim1][delim2][dimen]
Description
\abovewithdelims is TeX's most general command for making fractions. Printing
a\abovewithdelims(]0.4pt b
produces the fraction where the separating bar has thickness 0.4pt and it has delimiter ( on the left and ] on the right. The delimiters are treated as if \left precedes the left delimiter and \right precedes the right delimiter, and hence the delimiters scale based on the size of the contained text. A null delimiter can be used by putting a period . in for one of the delimiters. Hence \above can be replicated as
\above = \abovewithdelims..
Where each is followed by a dimension.