A pretty-printer for Leo (code) files.
This pretty-printer produces text
in the form of msgp and msg-listp values.
The latter generally consist of lines of text;
that is always the case at the top level,
i.e. a Leo file is turned into a list of lines.
Some pretty-printing functions produce msgp values
that other pretty-printing functions
incorporate into larger text.
In the pretty-printing functions,
we consistently use the result names