[SOLVED] Opam: conditioning build-steps on package-versions?

[The latest opam beta 2.1.0 solves this problem.]
I’d like to condition build-steps on package-versions in opam, viz:

build: [
  ["./configure" "--prefix" prefix "-libdir" lib "-mandir" man]
  [make "world.opt" "all"]
  [make "-C" "testsuite" "clean" "all-tests"] {with-test & "ocaml" >= "4.10.0" }
  [make "-C" "test" "clean" "all"] {with-test & "ocaml" >= "4.10.0" }

where I’d like the tests to run, but only if ocaml is at least version 4.10.0. This doesn’t seem to work with opam (version 2.0.5`). I can’t find anything in the manual saying this should work, nor that it shouldn’t.

It does parse without error. Should this work? I searched opam-repository/packages, and find no nontrivial examples of with-test usage.

ETA: digging further, I find in opam’s tests, the syntax {ocaml:version > "z100"}. I tried a variant of this (with 4.07.0 as the switch): {with-test & ocaml:version >= "4.10.0" } (and also removing the with-test &) to no avail.
ETA2: ah, the previous ETA is from the current sources; so now working thru installing the dev version of opam.