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:".