[ANN] First release of pretty_expressive: A Pretty Expressive Printer

That look very nice.
I have a small nitpick. You seem to use <> to concatenate documents, but <> is the “not equal operator” in OCaml, which means that you cannot use it while your library is opened. Maybe a better choice would be ^^, because ^ is the string concatenation operator. Francois Pottier’s PPrint made this choice and I think its quite nice.