This module offers a high-level and functional interface to the Format module
of the OCaml standard library. It is a pretty-printing facility, i.e. it takes
as input some code represented as a tree and formats this code into the most
visually satisfying result, breaking and indenting lines of code where
appropriate.

Input data must be first modelled and converted into a tree using 3 kinds of
nodes:

atoms lists labelled nodes

Atoms represent any text that is guaranteed to be printed as-is. Lists can
model any sequence of items such as arrays of data or lists of definitions that
are labelled with something like "int main", "let x =" or "x:".