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 modeled and converted into a tree using 3
kinds of nodes:

atoms
lists
labeled 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 labeled with something like "int main", "let x
=" or "x:".