Add comment that causes segfault and opam lock

When this project is built with `dune build` (with the switch specified
in the opam lock file installed), it results the following segfault:

    ocamlopt lib/abt.cmxs (exit 2)
(cd _build/default && <snip>/.opam/4.12.0/bin/ocamlopt.opt -w @1..3@5..28@30..39@43@46..47@49..57@61..62-40 -strict-sequence -strict-formats -short-paths -keep-locs -g -shared -linkall -I lib -o lib/abt.cmxs lib/abt.cmxa)
Fatal error: exception End_of_file
    ocamlopt test/test.exe (exit 2)
(cd _build/default && <snip>/.opam/4.12.0/bin/ocamlopt.opt -w @1..3@5..28@30..39@43@46..47@49..57@61..62-40 -strict-sequence -strict-formats -short-paths -keep-locs -g -o test/test.exe <snip>/um-abt/_opam/lib/base/base_internalhash_types/base_internalhash_types.cmxa -I <snip>/um-abt/_opam/lib/base/base_internalhash_types <snip>/um-abt/_opam/lib/base/caml/caml.cmxa <snip>/um-abt/_opam/lib/sexplib0/sexplib0.cmxa <snip>/um-abt/_opam/lib/base/shadow_stdlib/shadow_stdlib.cmxa <snip>/um-abt/_opam/lib/base/base.cmxa -I <snip>/um-abt/_opam/lib/base <snip>/um-abt/_opam/lib/ppx_inline_test/config/inline_test_config.cmxa <snip>/um-abt/_opam/lib/jane-street-headers/jane_street_headers.cmxa <snip>/um-abt/_opam/lib/ppx_sexp_conv/runtime-lib/ppx_sexp_conv_lib.cmxa <snip>/um-abt/_opam/lib/ppx_compare/runtime-lib/ppx_compare_lib.cmxa <snip>/um-abt/_opam/lib/ppx_enumerate/runtime-lib/ppx_enumerate_lib.cmxa <snip>/um-abt/_opam/lib/ppx_hash/runtime-lib/ppx_hash_lib.cmxa <snip>/um-abt/_opam/lib/time_now/time_now.cmxa -I <snip>/um-abt/_opam/lib/time_now <snip>/um-abt/_opam/lib/ppx_inline_test/runtime-lib/ppx_inline_test_lib.cmxa <snip>/um-abt/_opam/lib/ppx_here/runtime-lib/ppx_here_lib.cmxa <snip>/um-abt/_opam/lib/ppx_expect/common/expect_test_common.cmxa <snip>/um-abt/_opam/lib/ppx_expect/config_types/expect_test_config_types.cmxa <snip>/um-abt/_opam/lib/ppx_expect/collector/expect_test_collector.cmxa -I <snip>/um-abt/_opam/lib/ppx_expect/collector <snip>/um-abt/_opam/lib/ppx_expect/config/expect_test_config.cmxa lib/abt.cmxa <snip>/.opam/4.12.0/lib/ocaml/unix.cmxa -I <snip>/.opam/4.12.0/lib/ocaml <snip>/um-abt/_opam/lib/qcheck-core/qcheck_core.cmxa <snip>/um-abt/_opam/lib/qcheck-core/runner/qcheck_runner.cmxa <snip>/um-abt/_opam/lib/stdlib-shims/stdlib_shims.cmxa <snip>/um-abt/_opam/lib/ounit2/advanced/oUnitAdvanced.cmxa <snip>/um-abt/_opam/lib/ounit2/oUnit.cmxa <snip>/um-abt/_opam/lib/qcheck-ounit/qcheck_ounit.cmxa <snip>/um-abt/_opam/lib/qcheck/qcheck.cmxa <snip>/um-abt/_opam/lib/result/result.cmxa <snip>/um-abt/_opam/lib/ppx_deriving/runtime/ppx_deriving_runtime.cmxa test/example/example.cmxa test/.test.eobjs/native/dune__exe.cmx test/.test.eobjs/native/dune__exe__Abt_gen.cmx test/.test.eobjs/native/dune__exe__Test.cmx)
Fatal error: exception End_of_file

lib/abt.ml

@@ -220,4 +220,6 @@ module Make (O : Operator) = struct
     free Var.Set.empty t
 
   let is_closed : t -> bool = fun t -> Var.Set.is_empty (free_vars t)
+
+  (* let unify : t -> t -> t *  *)
 end

um-abt.opam.locked

@@ -0,0 +1,38 @@
+opam-version: "2.0"
+synopsis: "An OCaml library implementing abstract binding trees (ABTs)"
+description: """
+An ABT library consistent with the interface defined in Robert Harper's
+   'Practical Foundations for Programming Languages', using immutable
+   pointers to for varibale binding."""
+maintainer: ["shon.feder@gmail.com"]
+authors: ["Shon Feder"]
+license: "MIT"
+homepage: "https://github.com/shonfeder/um-abt"
+doc: "https://shonfeder.github.io/um-abt"
+bug-reports: "https://github.com/shonfeder/um-abt/issues"
+depends: [
+  "base-bigarray" {= "base"}
+  "base-threads" {= "base"}
+  "base-unix" {= "base"}
+  "dune" {= "2.8.5"}
+  "ocaml" {= "4.12.0"}
+  "ocaml-config" {= "2"}
+  "ocaml-system" {= "4.12.0"}
+]
+build: [
+  ["dune" "subst"] {dev}
+  [
+    "dune"
+    "build"
+    "-p"
+    name
+    "-j"
+    jobs
+    "@install"
+    "@runtest" {with-test}
+    "@doc" {with-doc}
+  ]
+]
+dev-repo: "git+https://github.com/shonfeder/um-abt.git"
+name: "um-abt"
+version: "dev"