OCaml syntax extension for object-oriented visitors

 Visitors is a syntax extension for the OCaml programming language. It
 allows you to annotate your type definitions, such that a visitor class
 will be automatically generated. This visitor class contains methods
 for the traversal of your data structure, like iter, map, fold, etc. It
 is then easy to override these methods for your needs. This is very
 convenient when programming with complex algebraic data structures.