[documentation]: document seq tactic

doc/tactics/seq.rst

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+========================================================================
+Tactic: ``seq``
+========================================================================
+
+The ``seq`` tactic applies to program-logic goals by splitting the
+program(s) under consideration into two consecutive parts.
+
+Applying ``seq`` allows one to decompose such a goal into two separate
+program-logic obligations by splitting the program(s) at a chosen point.
+The first obligation establishes that the initial precondition ensures
+some intermediate assertion after execution of the first part. The second
+obligation then shows that, assuming this intermediate assertion, execution
+of the second part entails the desired postcondition.
+
+The ``seq`` tactic requires the intermediate assertion to be supplied by
+the user, it is not inferred automatically.
+
+.. contents::
+   :local:
+
+------------------------------------------------------------------------
+Variant: Hoare logic
+------------------------------------------------------------------------
+
+.. admonition:: Syntax
+
+   ``seq {codepos} : ({formula})``
+
+The ``{codepos}`` specifies the position in the program where the split
+should occur. The formula ``{formula}`` is the intermediate assertion that
+must hold after execution of the first part of the program.
+
+Applying this form of ``seq`` produces two Hoare-style proof obligations:
+one for the first segment, from the original precondition to the supplied
+intermediate assertion, and one for the remaining segment, from this
+assertion to the original postcondition.
+
+.. ecproof::
+   :title: Hoare logic example   
+
+   require import AllCore.
+
+   module M = {
+     proc add3(x : int) : int = {
+       var y : int;
+       y <- x + 1;
+       y <- y + 2;
+       return y;
+     }
+   }.
+
+   lemma add3_correct (n : int) :
+     hoare [M.add3 : x = n ==> res = n + 3].
+   proof.
+     proc.
+
+     (*$*) (* Split after the first command, and supply an intermediate assertion. *)
+     seq 1 : (y = n + 1).
+
+     - (* First part: y <- x + 1 *)
+       wp. skip. smt().
+
+     - (* Second part: y <- y + 2; return y *)
+       wp. skip. smt().
+   qed.
+
+------------------------------------------------------------------------
+Variant: Probabilistic relation Hoare logic (two-sided)
+------------------------------------------------------------------------
+
+.. admonition:: Syntax
+
+   ``seq {codepos} {codepos} : {formula}``
+
+Here, the first ``{codepos}`` specifies where to split the left-hand
+program, and the second {codepos} specifies where to split the right-hand
+program. The formula ``{formula}`` is the intermediate *relational* assertion
+that must hold between the two memories after execution of the first parts.
+
+Applying this form of ``seq`` produces two relational proof obligations:
+one for the initial segments of both programs, from the original precondition
+to the supplied intermediate assertion, and one for the remaining segments,
+from this assertion to the original postcondition.
+
+.. ecproof::
+   :title: Probabilistic relational Hoare logic example   
+
+   require import AllCore.
+
+   module L = {
+     proc inc_then_double(x : int) : int = {
+       var y, z : int;
+
+       y <- x + 1;
+       z <- y * 2;
+       return z;
+     }
+   }.
+
+   module R = {
+     proc keep_then_double_as_if_inc(x : int) : int = {
+       var y, z : int;
+
+       y <- x;
+       z <- (y + 1) * 2;
+       return z;
+     }
+   }.
+
+   lemma inc_then_double_equiv : equiv [
+     L.inc_then_double ~ R.keep_then_double_as_if_inc :
+       x{1} = x{2} ==> res{1} = res{2}
+   ].
+   proof.
+     proc.
+
+     (*$*) (* Split after the first command on both sides, and relate the intermediate states. *)
+     seq 1 1 : (y{1} = y{2} + 1).
+
+     - (* First parts: y <- x + 1   ~   y <- x *)
+       wp. skip. smt().
+
+     - (* Second parts: z <- ...; return z   ~   z <- ...; return z *)
+       wp. skip. smt().
+   qed.
+
+------------------------------------------------------------------------
+Variant: Probabilistic relation Hoare logic (one-sided)
+------------------------------------------------------------------------
+
+.. admonition:: Syntax
+
+   ``seq {side} {codepos} : (_: {formula} ==> {formula})``
+
+------------------------------------------------------------------------
+Variant: Probabilistic Hoare logic
+------------------------------------------------------------------------
+
+.. admonition:: Syntax
+
+   ``seq {codepos} : {formula} {formula|_} {formula|_} {formula|_} {formula|_} {formula}?``
+
+Here:
+
+- ``{codepos}`` specifies where the program is split into a first and a
+  second part.
+- The first ``{formula}`` is the probabilistic intermediate assertion,
+  used to connect the probability reasoning between the two parts of the
+  program.
+- The four arguments ``{formula|_}`` are the real probability bounds required
+  by the probabilistic sequencing rule. Each may be given explicitly, or left
+  as ``_`` to let EasyCrypt infer or simplify it.
+- The last optional ``{formula}`` is a non-probabilistic intermediate
+  assertion that is required to hold after execution of the first part.
+  When provided, it is added to the preconditions of the subgoals targeting
+  the second part of the program.
+
+The four real coefficients supplied to ``seq`` correspond to the two possible
+outcomes of the intermediate probabilistic assertion:
+
+- The **first coefficient** bounds the probability that the intermediate
+  assertion holds after executing the first part of the program.
+
+- The **second coefficient** bounds the probability of reaching the final
+  postcondition after executing the second part, assuming the intermediate
+  assertion holds.
+
+- The **third coefficient** bounds the probability that the intermediate
+  assertion does *not* hold after the first part.
+
+- The **fourth coefficient** bounds the probability of reaching the final
+  postcondition after the second part, assuming the intermediate assertion
+  does not hold.
+
+These coefficients are then combined in the final arithmetic goal to obtain
+the probability bound of the original judgement.
+
+When `seq` is applied to a probabilistic Hoare-logic goal, EasyCrypt splits
+the program into two parts and generates several proof obligations that
+justify how the probability of the whole program is obtained from the two
+fragments.
+
+Concretely, the tactic produces the following kinds of goals:
+
+- **A Hoare goal for the optional pure assertion**. 
+  If a final (non-probabilistic) intermediate assertion is provided, the
+  first goal is to prove that the first part of the program establishes this
+  assertion from the current precondition. If no assertion is given, it is
+  taken to be `true`, and this goal is trivial.
+
+- **A probabilistic goal for the first part**.
+  A goal is generated to bound the probability that the first part of the
+  program establishes the intermediate probabilistic assertion.
+
+- **Probabilistic goals for the second part under the intermediate assertion**.
+  The second part of the program is then considered assuming that the
+  intermediate probabilistic assertion holds (and also assuming the optional
+  pure assertion, if one was provided). A goal is generated to bound the
+  probability of reaching the final postcondition in this case.
+
+- **Probabilistic goals for the second part under the negation of the intermediate assertion**.
+  A symmetric goal is generated for the case where the intermediate
+  probabilistic assertion does *not* hold after the first part.
+
+- **A final logical side-condition combining the bounds**  
+  Finally, EasyCrypt generates a pure arithmetic obligation stating that the
+  probability bounds supplied to ``seq`` correctly combine to give the overall
+  probability bound of the original goal.
+
+.. ecproof::
+   :title: Probabilistic Hoare logic example   
+
+   require import AllCore Distr DInterval.
+
+   module M = {
+     proc two_steps() : bool = {
+       var x, y : int;
+
+       x <$ [0..2];
+       y <$ [0..2];
+       return (x <> 0) /\ (y = 2);
+     }
+   }.
+
+   lemma two_steps_pr :
+     phoare [ M.two_steps : true ==> res ] = (2%r / 9%r).
+   proof.
+     proc.
+
+     (*$*)
+     (* Split after the first sampling.
+        Probabilistic intermediate assertion: x <> 0.
+        Extra pure assertion: true (trivial here). *)
+     seq 1 :
+       (x <> 0)
+       (2%r/3%r)   (* probability that x <> 0 *)
+       (1%r/3%r)   (* probability that x <> 0 /\ y = 2 | x <> 0 *)
+       (1%r/3%r)   (* probability that x = 0 *)
+       (0%r)       (* probability that x <> 0 /\ y = 2 | x = 0 *)
+       (true).
+
+     - (* Hoare goal: first part establishes the extra assertion *)
+       by auto.
+
+     - 
+       rnd (predC1 0). skip=> /=. split.
+       - rewrite mu_not.
+         rewrite dinter_ll //=.
+         rewrite dinter1E /=.
+         done.
+       - smt().
+
+     - rnd (pred1 2). skip=> &hr /= *. split.
+       - by rewrite dinter1E. 
+       - smt().
+
+     - rnd pred0. skip=> &hr /= *. split.
+       - by rewrite mu0.
+       - smt().
+
+     - done.
+   qed.
+
+------------------------------------------------------------------------
+Variant: Expectation Hoare logic
+------------------------------------------------------------------------
+
+.. admonition:: Syntax
+
+   ``seq {codepos} : {formula}``