$ sudo USE="-ocamlopt" emerge -a1 dev-ml/csexp

These are the packages that would be merged, in order:

Calculating dependencies... done!
[ebuild  N    ~] dev-ml/csexp-1.3.2  USE="-ocamlopt -test"

Would you like to merge these packages? [Yes/No]

>>> Verifying ebuild manifests

>>> Emerging (1 of 1) dev-ml/csexp-1.3.2::gentoo
 * csexp-1.3.2.tbz BLAKE2B SHA512 size ;-) ...                                                                                                                                                                                         [ ok ]
>>> Unpacking source...
>>> Unpacking csexp-1.3.2.tbz to /var/tmp/portage/dev-ml/csexp-1.3.2/work
>>> Source unpacked in /var/tmp/portage/dev-ml/csexp-1.3.2/work
>>> Preparing source in /var/tmp/portage/dev-ml/csexp-1.3.2/work/csexp-1.3.2 ...
>>> Source prepared.
>>> Configuring source in /var/tmp/portage/dev-ml/csexp-1.3.2/work/csexp-1.3.2 ...
>>> Source configured.
>>> Compiling source in /var/tmp/portage/dev-ml/csexp-1.3.2/work/csexp-1.3.2 ...
File "src/csexp.mli", line 36, characters 54-62:
36 |   val parse_string : string -> (Sexp.t, int * string) Result.t
                                                           ^^^^^^^^
Error: Unbound type constructor Result.t
Done: 23/39 (jobs: 1) * ERROR: dev-ml/csexp-1.3.2::gentoo failed (compile phase):
 *   (no error message)
 *