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In this changelog, we document "large-ish" changes to Iris that affect even the
way the logic is used on paper.  We also mention some significant changes in the
Coq development, but not every API-breaking change is listed.  Changes marked
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`[#]` still need to be ported to the Iris Documentation LaTeX file(s).
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## Iris master

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Changes in and extensions of the theory:

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* [#] Change in the definition of WP, so that there is a fancy update between
  the quantification over the next states and the later modality. This makes it
  possible to prove more powerful lifting lemmas: The new versions feature an
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  "update that takes a step".
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* [#] Weaken the semantics of CAS in heap_lang to be efficiently implementable:
  CAS may only be used to compare "unboxed" values that can be represented in a
  single machine word.
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* [#] Add weakest preconditions for total program correctness.
* [#] "(Potentially) stuck" weakest preconditions are no longer considered
  experimental.
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* [#] The adequacy statement for weakest preconditions now also involves the
  final state.
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* [#] Add the notion of an "observation" to the language interface, so that
  every reduction step can optionally be marked with an event, and an execution
  trace has a matching list of events.  Change WP so that it is told the entire
  future trace of observations from the beginning.  Use this in heap_lang to
  implement prophecy variables.
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* [#] The Löb rule is now a derived rule; it follows from later-intro, later
  being contractive and the fact that we can take fixpoints of contractive
  functions.
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* [#] Add atomic updates and logically atomic triples, including tactic support.
  See `heap_lang/lib/increment.v` for an example.
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* [#] heap_lang now uses right-to-left evaluation order. This makes it
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  significantly easier to write specifications of curried functions.
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* [#] heap_lang values are now injected in heap_lang expressions via a specific
  constructor of the expr inductive type. This simplifies much the tactical
  infrastructure around the language. In particular, this allow us to get rid
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  the reflection mechanism that was needed for proving closedness, atomicity and
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  "valueness" of a term. The price to pay is the addition of new
  "administrative" reductions in the operational semantics of the language.
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* [#] Extend the state interpretation with a natural number that keeps track of
  the number of forked-off threads, and have a global fixed proposition that
  describes the postcondition of each forked-off thread (instead of it being
  `True`). Additionally, there is a stronger variant of the adequacy theorem
  that allows to make use of the postconditions of the forked-off threads.
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Changes in Coq:

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* An all-new generalized proof mode that abstracts away from Iris!  See
  <http://iris-project.org/mosel/> for the corresponding paper.  Major new
  features:
  - The proof mode can now be used with logics derived from Iris (like iGPS),
    with non-step-indexed logics and even with non-affine (i.e., linear) logics.
  - `iModIntro` is more flexible and more powerful, it now also subsumes
    `iNext` and `iAlways`.
  - General infrastructure for deriving a logic for monotone predicates over
    an existing logic (see the paper for more details).
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    Developments instantiating the proof mode typeclasses may need significant
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  changes.  For developments just using the proof mode tactics, porting should
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  not be too much effort.  Notable things to port are:
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  - All the BI laws moved from the `uPred` module to the `bi` module.  For
    example, `uPred.later_equivI` became `bi.later_equivI`.
  - Big-ops are automatically imported, imports of `iris.base_logic.big_op` have
    to be removed.
  - The ⊢ notation can sometimes infer different (but convertible) terms when
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    searching for the BI to use, which (due to Coq limitations) can lead to
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    failing rewrites, in particular when rewriting at function types.
* The `iInv` tactic can now be used without the second argument (the name for
  the closing update).  It will then instead add the obligation to close the
  invariant to the goal.
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* Added support for defining derived connectives involving n-ary binders using
  telescopes.
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* The proof mode now more consistently "prettifies" the goal after each tactic.
  Prettification also simplifies some BI connectives, like conditional
  modalities and telescope quantifiers.
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* Improved pretty-printing of Iris connectives (in particular WP and fancy
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  updates) when Coq has to line-wrap the output.  This goes hand-in-hand with an
  improved test suite that also tests pretty-printing.
* Added a `gmultiset` RA.
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* Rename `timelessP` -> `timeless` (projection of the `Timeless` class)
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* The CMRA axiom `cmra_extend` is now stated in `Type`, using `sigT` instead of
  in `Prop` using `exists`. This makes it possible to define the function space
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  CMRA even for an infinite domain.
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* Rename proof mode type classes for laters:
  - `IntoLaterN``MaybeIntoLaterN` (this one _may_ strip a later)
  - `IntoLaterN'``IntoLaterN` (this one _should_ strip a later)
  - `IntoLaterNEnv``MaybeIntoLaterNEnv`
  - `IntoLaterNEnvs``MaybeIntoLaterNEnvs`
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* Rename:
  - `frag_auth_op``frac_auth_frag_op`
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  - `cmra_opM_assoc``cmra_op_opM_assoc`
  - `cmra_opM_assoc_L``cmra_op_opM_assoc_L`
  - `cmra_opM_assoc'``cmra_opM_opM_assoc`
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* `namespaces` has been moved to std++.
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* Changed `IntoVal` to be directly usable for rewriting `e` into `of_val v`, and
  changed `AsVal` to be usable for rewriting via the `[v <-]` destruct pattern.
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* `wp_fork` is now written in curried form.
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* `PureExec`/`wp_pure` now supports taking multiple steps at once.
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* A new tactic, `wp_pures`, executes as many pure steps as possible, excluding
  steps that would require unlocking subterms. Every impure wp_ tactic executes
  this tactic before doing anything else.
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* Add `big_sepM_insert_acc`.
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* The `_strong` lemmas (e.g. `own_alloc_strong`) work for all infinite
  sets, instead of just for cofinite sets. The versions with cofinite
  sets have been renamed to use the `_cofinite` suffix.
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* Remove locked value lambdas. The value scope notations `rec: f x := e` and
  `(λ: x, e)` no longer add a `locked`. Instead, we made the `wp_` tactics
  smarter to no longer unfold lambdas/recs that occur behind definitions.
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## Iris 3.1.0 (released 2017-12-19)
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Changes in and extensions of the theory:

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* Define `uPred` as a quotient on monotone predicates `M -> SProp`.
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* Get rid of some primitive laws; they can be derived:
  `True ⊢ □ True` and `□ (P ∧ Q) ⊢ □ (P ∗ Q)`
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* Camera morphisms have to be homomorphisms, not just monotone functions.
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* Add a proof that `f` has a fixed point if `f^k` is contractive.
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* Constructions for least and greatest fixed points over monotone predicates
  (defined in the logic of Iris using impredicative quantification).
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* Add a proof of the inverse of `wp_bind`.
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* [Experimental feature] Add new modality: ■ ("plainly").
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* [Experimental feature] Support verifying code that might get stuck by
  distinguishing "non-stuck" vs. "(potentially) stuck" weakest
  preconditions. (See [Swasey et al., OOPSLA '17] for examples.) The non-stuck
  `WP e @ E {{ Φ }}` ensures that, as `e` runs, it does not get stuck. The stuck
  `WP e @ E ?{{ Φ }}` ensures that, as usual, all invariants are preserved while
  `e` runs, but it permits execution to get stuck. The former implies the
  latter. The full judgment is `WP e @ s; E {{ Φ }}`, where non-stuck WP uses
  *stuckness bit* `s = NotStuck` while stuck WP uses `s = MaybeStuck`.
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Changes in Coq:

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* Move the `prelude` folder to its own project:
  [coq-std++](https://gitlab.mpi-sws.org/robbertkrebbers/coq-stdpp)
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* Some extensions/improvements of heap_lang:
  - Improve handling of pure (non-state-dependent) reductions.
  - Add fetch-and-add (`FAA`) operation.
  - Add syntax for all Coq's binary operations on `Z`.
* Generalize `saved_prop` to let the user choose the location of the type-level
  later.  Rename the general form to `saved_anything`.  Provide `saved_prop` and
  `saved_pred` as special cases.
* Improved big operators:
  + They are no longer tied to cameras, but work on any monoid
  + The version of big operations over lists was redefined so that it enjoys
    more definitional equalities.
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* Rename some things and change notation:
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  - The unit of a camera: `empty` -> `unit`, `∅` -> `ε`
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  - Disjointness: `⊥` -> `##`
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  - A proof mode type class `IntoOp` -> `IsOp`
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  - OFEs with all elements being discrete: `Discrete` -> `OfeDiscrete`
  - OFE elements whose equality is discrete: `Timeless` -> `Discrete`
  - Timeless propositions: `TimelessP` -> `Timeless`
  - Camera elements such that `core x = x`: `Persistent` -> `CoreId`
  - Persistent propositions: `PersistentP` -> `Persistent`
  - The persistent modality: `always` -> `persistently`
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  - Adequacy for non-stuck weakestpre: `adequate_safe` -> `adequate_not_stuck`
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  - Consistently SnakeCase identifiers:
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    + `CMRAMixin` -> `CmraMixin`
    + `CMRAT` -> `CmraT`
    + `CMRATotal` -> `CmraTotal`
    + `CMRAMorphism` -> `CmraMorphism`
    + `CMRADiscrete` -> `CmraDiscrete`
    + `UCMRAMixin` -> `UcmraMixin`
    + `UCMRAT` -> `UcmraT`
    + `DRAMixin` -> `DraMixin`
    + `DRAT` -> `DraT`
    + `STS` -> `Sts`
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  - Many lemmas also changed their name.  `always_*` became `persistently_*`,
    and furthermore: (the following list is not complete)
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    + `impl_wand` -> `impl_wand_1` (it only involves one direction of the
      equivalent)
    + `always_impl_wand` -> `impl_wand`
    + `always_and_sep_l` -> `and_sep_l`
    + `always_and_sep_r` -> `and_sep_r`
    + `always_sep_dup` -> `sep_dup`
    + `wand_impl_always` -> `impl_wand_persistently` (additionally,
      the direction of this equivalence got swapped for consistency's sake)
    + `always_wand_impl` -> `persistently_impl_wand` (additionally, the
      direction of this equivalence got swapped for consistency's sake)
  The following `sed` snippet should get you most of the way:
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```
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sed 's/\bPersistentP\b/Persistent/g; s/\bTimelessP\b/Timeless/g; s/\bCMRADiscrete\b/CmraDiscrete/g; s/\bCMRAT\b/CmraT/g; s/\bCMRAMixin\b/CmraMixin/g; s/\bUCMRAT\b/UcmraT/g; s/\bUCMRAMixin\b/UcmraMixin/g; s/\bSTS\b/Sts/g' -i $(find -name "*.v")
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```
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* `PersistentL` and `TimelessL` (persistence and timelessness of lists of
  propositions) are replaces by `TCForall` from std++.
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* Fix a bunch of consistency issues in the proof mode, and make it overall more
  usable.  In particular:
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  - All proof mode tactics start the proof mode if necessary; `iStartProof` is
    no longer needed and should only be used for building custom proof mode
    tactics.
  - Change in the grammar of specialization patterns: `>[...]` -> `[> ...]`
  - Various new specification patterns for `done` and framing.
  - There is common machinery for symbolic execution of pure reductions. This
    is provided by the type classes `PureExec` and `IntoVal`.
  - There is a new connective `tc_opaque`, which can be used to make definitions
    opaque for type classes, and thus opaque for most tactics of the proof
    mode.
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  - Define Many missing type class instances for distributing connectives.
  - Implement the tactics `iIntros (?)` and `iIntros "!#"` (i.e. `iAlways`)
    using type classes. This makes them more generic, e.g., `iIntros (?)` also
    works when the universal quantifier is below a modality, and `iAlways` also
    works for the plainness modality.  A breaking change, however, is that these
    tactics now no longer work when the universal quantifier or modality is
    behind a type class opaque definition.  Furthermore, this can change the
    name of anonymous identifiers introduced with the "%" pattern.
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* Make `ofe_fun` dependently typed, subsuming `iprod`.  The latter got removed.
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* Define the generic `fill` operation of the `ectxi_language` construct in terms
  of a left fold instead of a right fold. This gives rise to more definitional
  equalities.
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* The language hierarchy (`language`, `ectx_language`, `ectxi_language`) is now
  fully formalized using canonical structures instead of using a mixture of
  type classes and canonical structures. Also, it now uses explicit mixins. The
  file `program_logic/ectxi_language` contains some documentation on how to
  setup Iris for your language.
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* Restore the original, stronger notion of atomicity alongside the weaker
  notion. These are `Atomic a e` where the stuckness bit `s` indicates whether
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  expression `e` is weakly (`a = WeaklyAtomic`) or strongly
  (`a = StronglyAtomic`) atomic.
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* Various improvements to `solve_ndisj`.
* Use `Hint Mode` to prevent Coq from making arbitrary guesses in the presence
  of evars, which often led to divergence. There are a few places where type
  annotations are now needed.
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* The rules `internal_eq_rewrite` and `internal_eq_rewrite_contractive` are now
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  stated in the logic, i.e., they are `iApply`-friendly.
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## Iris 3.0.0 (released 2017-01-11)
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* There now is a deprecation process.  The modules `*.deprecated` contain
  deprecated notations and definitions that are provided for backwards
  compatibility and will be removed in a future version of Iris.
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* View shifts are radically simplified to just internalize frame-preserving
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  updates.  Weakestpre is defined inside the logic, and invariants and view
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  shifts with masks are also coded up inside Iris.  Adequacy of weakestpre is
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  proven in the logic. The old ownership of the entire physical state is
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  replaced by a user-selected predicate over physical state that is maintained
  by weakestpre.
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* Use OFEs instead of COFEs everywhere.  COFEs are only used for solving the
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  recursive domain equation.  As a consequence, CMRAs no longer need a proof of
  completeness.  (The old `cofeT` is provided by `algebra.deprecated`.)
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* Implement a new agreement construction.  Unlike the old one, this one
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  preserves discreteness.  dec_agree is thus no longer needed and has been moved
  to algebra.deprecated.
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* Renaming and moving things around: uPred and the rest of the base logic are in
  `base_logic`, while `program_logic` is for everything involving the general
  Iris notion of a language.
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* Renaming in prelude.list: Rename `prefix_of` -> `prefix` and `suffix_of` ->
  `suffix` in lemma names, but keep notation ``l1 `prefix_of` l2`` and ``l1
  `suffix_of` l2``.  `` l1 `sublist` l2`` becomes ``l1 `sublist_of` l2``. Rename
  `contains` -> `submseteq` and change `` l1 `contains` l2`` to ``l1 ⊆+ l2``.
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* Slightly weaker notion of atomicity: an expression is atomic if it reduces in
  one step to something that does not reduce further.
* Changed notation for embedding Coq assertions into Iris.  The new notation is
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  ⌜φ⌝.  Also removed `=` and `⊥` from the Iris scope.  (The old notations are
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  provided in `base_logic.deprecated`.)
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* Up-closure of namespaces is now a notation (↑) instead of a coercion.
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* With invariants and the physical state being handled in the logic, there is no
  longer any reason to demand the CMRA unit to be discrete.
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* The language can now fork off multiple threads at once.
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* Local Updates (for the authoritative monoid) are now a 4-way relation with
  syntax-directed lemmas proving them.
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## Iris 2.0

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* [heap_lang] No longer use dependent types for expressions.  Instead, values
  carry a proof of closedness.  Substitution, closedness and value-ness proofs
  are performed by computation after reflecting into a term langauge that knows
  about values and closed expressions.
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* [program_logic/language] The language does not define its own "atomic"
  predicate.  Instead, atomicity is defined as reducing in one step to a value.
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* [program_logic] Due to a lack of maintenance and usefulness, lifting lemmas
  for Hoare triples are removed.

## Iris 2.0-rc2

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This version matches the final ICFP 2016 paper.
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* [algebra] Make the core of an RA or CMRA a partial function.
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* [program_logic/lifting] Lifting lemmas no longer round-trip through a
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  user-chosen predicate to define the configurations we can reduce to; they
  directly relate to the operational semantics.  This is equivalent and
  much simpler to read.
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## Iris 2.0-rc1

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This is the Coq development and Iris Documentation as submitted to ICFP 2016.