Adjust README for the symbol ADT.

README.md

 # IRIS EXAMPLES
 
-Some example verification demonstrating the use of Iris.
+This is a fork of the
+[iris-examples](https://gitlab.mpi-sws.org/FP/iris-examples.git)
+repository formalizing the safety proof sketched in Derek Dreyer's
+POPL '18 talk.
 
 ## Prerequisites
 
@@ -29,9 +32,54 @@ To update, do `git pull`.  After an update, the development may fail to compile
 because of outdated dependencies.  To fix that, please run `opam update`
 followed by `make build-dep`.
 
+## Derek's POPL '18 talk
+
+This repository contains the safety proof sketched in Derek Dreyer's
+POPL '18 talk. It is derived from [earlier
+work](https://gitlab.mpi-sws.org/FP/iris-examples.git) accompanying
+the [Iris proof mode
+paper](http://doi.acm.org/10.1145/3093333.3009855).
+
+The language, type system, and logical relation we use are defined in
+
+	theories/logrel/F_mu_ref_conc/lang.v
+	theories/logrel/F_mu_ref_conc/typing.v
+	theories/logrel/F_mu_ref_conc/logrel_unary.v
+
+The language is System F with recursive types, references,
+concurrency, and an unsafe expression `assert e` that gets stuck
+unless `e` evaluates to `true`. (An additional file,
+`theories/logrel/F_mu_ref_conc/rules.v`, derives some Iris proof rules
+from the language's operational semantics.)
+
+The relevant metatheorems are proved in
+
+	theories/logrel/F_mu_ref_conc/fundamental_unary.v
+	theories/logrel/F_mu_ref_conc/soundness_unary.v
+
+These are the fundamental theorem of logical relations and a semantic
+proof of type soundness.
+
+The symbol ADT is verified in
+
+	theories/logrel/F_mu_ref_conc/examples/symbol.v
+
+The goal is to show that a syntactically unsafe implementation of
+symbols is semantically safe:
+
+	pack <int,
+		let c = ref 0 in
+		<λ_. c++,
+		 λ_. assert(s < *c)>
+	as ∃α. (unit → α) × (α → unit)
+	∈ E[∃α. (unit → α) × (α → unit)]
+
+This is lemma `type_symbol` in that file.
+
 ## Case Studies
 
-This repository contains the following case studies:
+This repository also contains several case studies, disabled in
+`_CoqProject`:
 
 * [barrier](theories/barrier): Implementation and proof of a barrier as
   described in ["Higher-Order Ghost State"](http://doi.acm.org/10.1145/2818638).
@@ -56,6 +104,16 @@ This repository contains the following case studies:
 * [lecture-notes](theories/lecture_notes): Coq examples for the
   [Iris lecture notes](http://iris-project.org/tutorial-material.html).
 
+If you are interested in these, we recommend that you find them in
+[the original iris-examples
+repository](https://gitlab.mpi-sws.org/FP/iris-examples.git). To
+verify safety for the symbol ADT, we extended the language in
+`F_mu_rec_conc` with existential types, but we neither updated that
+language's binary logical relation nor proved that relation compatible
+with `pack` and `unpack`. This means that examples that rely on the
+binary logical relation for `F_mu_rec_conc` no longer compile.
+(Everything else would compile if enabled in `_CoqProject`.)
+
 ## For Developers: How to update the Iris dependency
 
 * Do the change in Iris, push it.

_CoqProject

 -Q theories iris_examples
 -arg -w -arg -notation-overridden,-redundant-canonical-projection,-several-object-files
-theories/barrier/proof.v
-theories/barrier/specification.v
-theories/barrier/barrier.v
-theories/barrier/protocol.v
-theories/barrier/example_client.v
-theories/barrier/example_joining_existentials.v
+#theories/barrier/proof.v
+#theories/barrier/specification.v
+#theories/barrier/barrier.v
+#theories/barrier/protocol.v
+#theories/barrier/example_client.v
+#theories/barrier/example_joining_existentials.v
 
-theories/lecture_notes/coq_intro_example_1.v
+#theories/lecture_notes/coq_intro_example_1.v
 
-theories/spanning_tree/graph.v
-theories/spanning_tree/mon.v
-theories/spanning_tree/spanning.v
-theories/spanning_tree/proof.v
+#theories/spanning_tree/graph.v
+#theories/spanning_tree/mon.v
+#theories/spanning_tree/spanning.v
+#theories/spanning_tree/proof.v
 
 theories/logrel/prelude/base.v
-theories/logrel/stlc/lang.v
-theories/logrel/stlc/typing.v
-theories/logrel/stlc/rules.v
-theories/logrel/stlc/logrel.v
-theories/logrel/stlc/fundamental.v
-theories/logrel/stlc/soundness.v
-theories/logrel/F_mu/lang.v
-theories/logrel/F_mu/typing.v
-theories/logrel/F_mu/rules.v
-theories/logrel/F_mu/logrel.v
-theories/logrel/F_mu/fundamental.v
-theories/logrel/F_mu/soundness.v
-theories/logrel/F_mu_ref/lang.v
-theories/logrel/F_mu_ref/typing.v
-theories/logrel/F_mu_ref/rules.v
-theories/logrel/F_mu_ref/rules_binary.v
-theories/logrel/F_mu_ref/logrel.v
-theories/logrel/F_mu_ref/logrel_binary.v
-theories/logrel/F_mu_ref/fundamental.v
-theories/logrel/F_mu_ref/fundamental_binary.v
-theories/logrel/F_mu_ref/soundness.v
-theories/logrel/F_mu_ref/context_refinement.v
-theories/logrel/F_mu_ref/soundness_binary.v
+#theories/logrel/stlc/lang.v
+#theories/logrel/stlc/typing.v
+#theories/logrel/stlc/rules.v
+#theories/logrel/stlc/logrel.v
+#theories/logrel/stlc/fundamental.v
+#theories/logrel/stlc/soundness.v
+#theories/logrel/F_mu/lang.v
+#theories/logrel/F_mu/typing.v
+#theories/logrel/F_mu/rules.v
+#theories/logrel/F_mu/logrel.v
+#theories/logrel/F_mu/fundamental.v
+#theories/logrel/F_mu/soundness.v
+#theories/logrel/F_mu_ref/lang.v
+#theories/logrel/F_mu_ref/typing.v
+#theories/logrel/F_mu_ref/rules.v
+#theories/logrel/F_mu_ref/rules_binary.v
+#theories/logrel/F_mu_ref/logrel.v
+#theories/logrel/F_mu_ref/logrel_binary.v
+#theories/logrel/F_mu_ref/fundamental.v
+#theories/logrel/F_mu_ref/fundamental_binary.v
+#theories/logrel/F_mu_ref/soundness.v
+#theories/logrel/F_mu_ref/context_refinement.v
+#theories/logrel/F_mu_ref/soundness_binary.v
 theories/logrel/F_mu_ref_conc/lang.v
 theories/logrel/F_mu_ref_conc/rules.v
 theories/logrel/F_mu_ref_conc/typing.v
 theories/logrel/F_mu_ref_conc/logrel_unary.v
 theories/logrel/F_mu_ref_conc/fundamental_unary.v
-theories/logrel/F_mu_ref_conc/rules_binary.v
+theories/logrel/F_mu_ref_conc/soundness_unary.v
+#theories/logrel/F_mu_ref_conc/rules_binary.v
 #theories/logrel/F_mu_ref_conc/logrel_binary.v
 #theories/logrel/F_mu_ref_conc/fundamental_binary.v
-theories/logrel/F_mu_ref_conc/soundness_unary.v
 #theories/logrel/F_mu_ref_conc/context_refinement.v
 #theories/logrel/F_mu_ref_conc/soundness_binary.v
-theories/logrel/F_mu_ref_conc/examples/lock.v
+#theories/logrel/F_mu_ref_conc/examples/lock.v
 #theories/logrel/F_mu_ref_conc/examples/counter.v
 #theories/logrel/F_mu_ref_conc/examples/stack/stack_rules.v
 #theories/logrel/F_mu_ref_conc/examples/stack/CG_stack.v
 #theories/logrel/F_mu_ref_conc/examples/stack/FG_stack.v
 #theories/logrel/F_mu_ref_conc/examples/stack/refinement.v
+theories/logrel/F_mu_ref_conc/examples/symbol.v