Install libppx-visitors-ocaml-dev by entering the following commands in the terminal:
sudo apt update sudo apt install libppx-visitors-ocaml-dev
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.