https://cp-algorithms.com/algebra/continued-fractions.html#linear-fractional-transformations

It looks like $x_k-z_ka_k$ was mistakenly replaced with $x_k-y_ka_k$ in the middle of an equation. The code should also be revised to be consistent with the variables in the explanation.

This commit from #962 fixes the code.

(Also, #962 (comment) remains unresolved.)