Thanks to MATLAB's unique matrix access capabilities, the lines colored red can be written in a single line.

In the i$-$tb row block, group the elements of M and make submatrices.

$\{\begin{array}{ccc}\text{:}& ifM\text{'}si-th\text{r}\text{o}\text{w}is\text{f}\text{i}\text{l}\text{l}\text{e}\text{d}& \end{array}$

$M\text{'}slst$ row $=$ a row with all zeros

end

A is M$\text{'}$s $2nd,3rd,$dots,$i-$th row.

B is M$\text{'}$s $1$st$,2nd,$dots., $($i$-1)-$th row.

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