liblwt-ocaml-dev 2.7.1-7ubuntu2 (amd64 binary) in ubuntu focal

 Lwt is a library for cooperative threads implemented in a monadic style.
 Unlike preemptive threads, cooperative threads do not use a scheduler to
 distribute processor time between threads. Instead, each thread must
 announce that it wants to let the others run.
 .
 This package contains all the development stuff you need to use the
 core Lwt OCaml library in your programs.