libextlib-ocaml-dev binary package in Ubuntu Focal amd64

 ExtLib is a project aiming at providing a complete - yet small -
 standard library for the OCaml programming language.
 .
 The purpose of this library is to add new functions to OCaml
 Standard Library modules, to modify some functions in order to
 get better performances or more safety (tail-recursive) but also
 to provide new modules which should be useful for the average
 OCaml programmer.
 .
 ExtLib contains modules implementing: enumeration over abstract
 collection of elements, efficient bit sets, dynamic arrays,
 references on lists, Unicode characters and UTF-8 encoded
 strings, additional and improved functions for hashtables,
 strings, lists and option types.

Source package