 Finite-horizon total progeny generating function

Count the ancestor and the next $n-1$ generations of a Galton-Watson process. Conditioning on the ancestor's offspring, their independent descendant trees each have the preceding finite-horizon probability generating function, proving the displayed recursion. If the count instead excludes the ancestor, the recursion is ${\hat{H}}_{n+1}(s)=G(s{\hat{H}}_n(s))$ with ${\hat{H}}_0=1$. The indexing convention must be specified.