\overwithdelims is one of TeX's general commands for making fractions. Printing
produces the fraction where 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 \over can be replicated as
\over = \overwithdelims..