  We give a framework for combining $n$ monads on the same category via
distributive laws satisfying Yang-Baxter equations, extending the classical
result of Barr and Wells which combines two monads via one distributive law. We
show that this corresponds to iterating $n$-times the process of taking the
2-category of monads in a 2-category, extending the result of Street
characterising distributive laws. We show that this framework can be used to
construct the free strict $n$-category monad on $n$-dimensional globular sets;
we first construct for each $i$ a monad for composition along bounding
$i$-cells, and then we show that the interchange laws define distributive laws
between these monads, satisfying the necessary Yang-Baxter equations.
