Base library and tools for ppx rewriters

A comprehensive toolbox for ppx development. It features:

• a OCaml AST / parser / pretty-printer snapshot,to create a full frontend independent of the version of OCaml;
• a library for library for ppx rewriters in general, and type-driven code generators in particular;
• a feature-full driver for OCaml AST transformers;
• a quotation mechanism allowing to write values representing the OCaml AST in the OCaml syntax;
• a generator of open recursion classes from type definitions.