This module is a binding to CRlibm, an efficient and proved correctly-rounded mathematical library. CRlibm is now superseded by MetaLibm but the latter requires some polishing and documentation. For the user convenience, this module embeds the relevant C code from the CRlibm Git repository.
The easier is to use opam:
opam install crlibm
See crlibm.mli, also available online.