I guess the page would benefit from listing the entire thing in the replacement code, instead of just the inner line. Then non-mathematical people like me would maybe have a chance at getting it without ramping up caffeine dosage to dangerous levels, so as to see the missing parts in the fabric of mathematical possibilities.
The result computed in the current iteration is a[i - j][j]. The result computed in the iteration before that is a[i - (j - 1)][j - 1].