val tag : unit B0_meta.keytag indicates the entity deals with OCaml code.
val requires : B00_ocaml.Lib_name.t list B0_meta.keyrequires on a build unit specifies the OCaml libraries needed to compile a unit.
val library : B00_ocaml.Lib_name.t B0_meta.keylibrary on a build unit specifies that the unit defines the library.