Tools for managing bracketing during pretty-printing based.
Based on an implementation I saw Gabriel Ebner use that feels complicated,
but it makes sense once you get going, and there's
no obvious solution that's better while preserving the generality this offers.
The general source of the complexity is that you want both the caller and the callee
to have control over whether or not something gets bracketed.
There's a working example in the pp function of /examples/aexp
The idea is that the pretty printing implementation for some type you have is a recursive
(or faux-recursive) function that returns elements wrapped in this Parenable type, and have
an associated number which lets the functions waiting on their return know whether or not
they should wrap the value they get in brackets.