  $\Pi_1^1$-ranks are a natural tool for studying coanalytic sets in
descriptive set theory. In the book "Classical descriptive set theory", Kechris
provided a technique to build $\Pi_1^1$-ranks using derivatives. In this note
we will prove a variant of this result that is applicable to the $\Gamma$-rank.
Some dynamical ranks, like the entropy rank can be stated in terms of the
$\Gamma$-rank.
