The ZigZag function applies a sequence of similarity transformations to the  mod m Matrix A to obtain the ZigZag form of A.

A ZigZag form is an almost block diagonal structure having fewer than  nonzero entries.

A ZigZag form can be used to obtain the Smith normal form and the Frobenius form of a Matrix. The Frobenius form of a Matrix is the same as the Frobenius form of its ZigZag form.

This command is part of the LinearAlgebra[Modular] package, so it can be used in the form ZigZag(..) only after executing the command with(LinearAlgebra[Modular]).  However, it can always be used in the form LinearAlgebra[Modular][ZigZag](..).