Mahler equations and rationality

We give another proof of a result of Adamczewski and Bell concerning Mahler equations: A formal power series satisfying a $p-$ and a $q-$Mahler equation over ${\mathbb C}(x)$ with multiplicatively independent positive integers $p$ and $q$ is a rational function. The proof presented here is self-contained and is essentially a compilation of proofs contained in the recent preprint "Consistent systems of linear differential and difference equations", arXiv:1605.02616 [math.CA], by the same authors.