The intent is that an 'a forest is a list of messages of type 'a. But messages can stand for a list of more precise messages, hence the structure is organised as a tree.

To build a trace incrementally, we use an intermediary data structure on which we can define an S-expression like language (like a simplified xml except the closing tags do not carry a name).

open a opens a tag with name a.

close closes the last open tag. It is the responsibility of the user to close all the tags.

leaf creates an empty tag with name a.