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+(*  Title:      eqsubst.ML
+    Author:     Lucas Dixon, University of Edinburgh
+    Modified:   Joshua Chen, University of Innsbruck
+
+Perform a substitution using an equation.
+
+This code is slightly modified from the original at Tools/eqsubst..ML,
+to incorporate auto-typechecking for type theory.
+*)
+
+signature EQSUBST =
+sig
+  type match =
+    ((indexname * (sort * typ)) list (* type instantiations *)
+      * (indexname * (typ * term)) list) (* term instantiations *)
+    * (string * typ) list (* fake named type abs env *)
+    * (string * typ) list (* type abs env *)
+    * term (* outer term *)
+
+  type searchinfo =
+    Proof.context
+    * int (* maxidx *)
+    * Zipper.T (* focusterm to search under *)
+
+  datatype 'a skipseq = SkipMore of int | SkipSeq of 'a Seq.seq Seq.seq
+
+  val skip_first_asm_occs_search: ('a -> 'b -> 'c Seq.seq Seq.seq) -> 'a -> int -> 'b -> 'c skipseq
+  val skip_first_occs_search: int -> ('a -> 'b -> 'c Seq.seq Seq.seq) -> 'a -> 'b -> 'c Seq.seq
+  val skipto_skipseq: int -> 'a Seq.seq Seq.seq -> 'a skipseq
+
+  (* tactics *)
+  val eqsubst_asm_tac: Proof.context -> int list -> thm list -> int -> tactic
+  val eqsubst_asm_tac': Proof.context ->
+    (searchinfo -> int -> term -> match skipseq) -> int -> thm -> int -> tactic
+  val eqsubst_tac: Proof.context ->
+    int list -> (* list of occurrences to rewrite, use [0] for any *)
+    thm list -> int -> tactic
+  val eqsubst_tac': Proof.context ->
+    (searchinfo -> term -> match Seq.seq) (* search function *)
+    -> thm (* equation theorem to rewrite with *)
+    -> int (* subgoal number in goal theorem *)
+    -> thm (* goal theorem *)
+    -> thm Seq.seq (* rewritten goal theorem *)
+
+  (* search for substitutions *)
+  val valid_match_start: Zipper.T -> bool
+  val search_lr_all: Zipper.T -> Zipper.T Seq.seq
+  val search_lr_valid: (Zipper.T -> bool) -> Zipper.T -> Zipper.T Seq.seq
+  val searchf_lr_unify_all: searchinfo -> term -> match Seq.seq Seq.seq
+  val searchf_lr_unify_valid: searchinfo -> term -> match Seq.seq Seq.seq
+  val searchf_bt_unify_valid: searchinfo -> term -> match Seq.seq Seq.seq
+end;
+
+structure EqSubst: EQSUBST =
+struct
+
+(* changes object "=" to meta "==" which prepares a given rewrite rule *)
+fun prep_meta_eq ctxt =
+  Simplifier.mksimps ctxt #> map Drule.zero_var_indexes;
+
+(* make free vars into schematic vars with index zero *)
+fun unfix_frees frees =
+   fold (K (Thm.forall_elim_var 0)) frees o Drule.forall_intr_list frees;
+
+
+type match =
+  ((indexname * (sort * typ)) list (* type instantiations *)
+   * (indexname * (typ * term)) list) (* term instantiations *)
+  * (string * typ) list (* fake named type abs env *)
+  * (string * typ) list (* type abs env *)
+  * term; (* outer term *)
+
+type searchinfo =
+  Proof.context
+  * int (* maxidx *)
+  * Zipper.T; (* focusterm to search under *)
+
+
+(* skipping non-empty sub-sequences but when we reach the end
+   of the seq, remembering how much we have left to skip. *)
+datatype 'a skipseq =
+  SkipMore of int |
+  SkipSeq of 'a Seq.seq Seq.seq;
+
+(* given a seqseq, skip the first m non-empty seq's, note deficit *)
+fun skipto_skipseq m s =
+  let
+    fun skip_occs n sq =
+      (case Seq.pull sq of
+        NONE => SkipMore n
+      | SOME (h, t) =>
+        (case Seq.pull h of
+          NONE => skip_occs n t
+        | SOME _ => if n <= 1 then SkipSeq (Seq.cons h t) else skip_occs (n - 1) t))
+  in skip_occs m s end;
+
+(* note: outerterm is the taget with the match replaced by a bound
+   variable : ie: "P lhs" beocmes "%x. P x"
+   insts is the types of instantiations of vars in lhs
+   and typinsts is the type instantiations of types in the lhs
+   Note: Final rule is the rule lifted into the ontext of the
+   taget thm. *)
+fun mk_foo_match mkuptermfunc Ts t =
+  let
+    val ty = Term.type_of t
+    val bigtype = rev (map snd Ts) ---> ty
+    fun mk_foo 0 t = t
+      | mk_foo i t = mk_foo (i - 1) (t $ (Bound (i - 1)))
+    val num_of_bnds = length Ts
+    (* foo_term = "fooabs y0 ... yn" where y's are local bounds *)
+    val foo_term = mk_foo num_of_bnds (Bound num_of_bnds)
+  in Abs ("fooabs", bigtype, mkuptermfunc foo_term) end;
+
+(* T is outer bound vars, n is number of locally bound vars *)
+(* THINK: is order of Ts correct...? or reversed? *)
+fun mk_fake_bound_name n = ":b_" ^ n;
+fun fakefree_badbounds Ts t =
+  let val (FakeTs, Ts, newnames) =
+    fold_rev (fn (n, ty) => fn (FakeTs, Ts, usednames) =>
+      let
+        val newname = singleton (Name.variant_list usednames) n
+      in
+        ((mk_fake_bound_name newname, ty) :: FakeTs,
+          (newname, ty) :: Ts,
+          newname :: usednames)
+      end) Ts ([], [], [])
+  in (FakeTs, Ts, Term.subst_bounds (map Free FakeTs, t)) end;
+
+(* before matching we need to fake the bound vars that are missing an
+   abstraction. In this function we additionally construct the
+   abstraction environment, and an outer context term (with the focus
+   abstracted out) for use in rewriting with RW_Inst.rw *)
+fun prep_zipper_match z =
+  let
+    val t = Zipper.trm z
+    val c = Zipper.ctxt z
+    val Ts = Zipper.C.nty_ctxt c
+    val (FakeTs', Ts', t') = fakefree_badbounds Ts t
+    val absterm = mk_foo_match (Zipper.C.apply c) Ts' t'
+  in
+    (t', (FakeTs', Ts', absterm))
+  end;
+
+(* Unification with exception handled *)
+(* given context, max var index, pat, tgt; returns Seq of instantiations *)
+fun clean_unify ctxt ix (a as (pat, tgt)) =
+  let
+    (* type info will be re-derived, maybe this can be cached
+       for efficiency? *)
+    val pat_ty = Term.type_of pat;
+    val tgt_ty = Term.type_of tgt;
+    (* FIXME is it OK to ignore the type instantiation info?
+       or should I be using it? *)
+    val typs_unify =
+      SOME (Sign.typ_unify (Proof_Context.theory_of ctxt) (pat_ty, tgt_ty) (Vartab.empty, ix))
+        handle Type.TUNIFY => NONE;
+  in
+    (case typs_unify of
+      SOME (typinsttab, ix2) =>
+        let
+          (* FIXME is it right to throw away the flexes?
+             or should I be using them somehow? *)
+          fun mk_insts env =
+            (Vartab.dest (Envir.type_env env),
+             Vartab.dest (Envir.term_env env));
+          val initenv =
+            Envir.Envir {maxidx = ix2, tenv = Vartab.empty, tyenv = typinsttab};
+          val useq = Unify.smash_unifiers (Context.Proof ctxt) [a] initenv
+            handle ListPair.UnequalLengths => Seq.empty
+              | Term.TERM _ => Seq.empty;
+          fun clean_unify' useq () =
+            (case (Seq.pull useq) of
+               NONE => NONE
+             | SOME (h, t) => SOME (mk_insts h, Seq.make (clean_unify' t)))
+            handle ListPair.UnequalLengths => NONE
+              | Term.TERM _ => NONE;
+        in
+          (Seq.make (clean_unify' useq))
+        end
+    | NONE => Seq.empty)
+  end;
+
+(* Unification for zippers *)
+(* Note: Ts is a modified version of the original names of the outer
+   bound variables. New names have been introduced to make sure they are
+   unique w.r.t all names in the term and each other. usednames' is
+   oldnames + new names. *)
+fun clean_unify_z ctxt maxidx pat z =
+  let val (t, (FakeTs, Ts, absterm)) = prep_zipper_match z in
+    Seq.map (fn insts => (insts, FakeTs, Ts, absterm))
+      (clean_unify ctxt maxidx (t, pat))
+  end;
+
+
+fun bot_left_leaf_of (l $ _) = bot_left_leaf_of l
+  | bot_left_leaf_of (Abs (_, _, t)) = bot_left_leaf_of t
+  | bot_left_leaf_of x = x;
+
+(* Avoid considering replacing terms which have a var at the head as
+   they always succeed trivially, and uninterestingly. *)
+fun valid_match_start z =
+  (case bot_left_leaf_of (Zipper.trm z) of
+    Var _ => false
+  | _ => true);
+
+(* search from top, left to right, then down *)
+val search_lr_all = ZipperSearch.all_bl_ur;
+
+(* search from top, left to right, then down *)
+fun search_lr_valid validf =
+  let
+    fun sf_valid_td_lr z =
+      let val here = if validf z then [Zipper.Here z] else [] in
+        (case Zipper.trm z of
+          _ $ _ =>
+            [Zipper.LookIn (Zipper.move_down_left z)] @ here @
+            [Zipper.LookIn (Zipper.move_down_right z)]
+        | Abs _ => here @ [Zipper.LookIn (Zipper.move_down_abs z)]
+        | _ => here)
+      end;
+  in Zipper.lzy_search sf_valid_td_lr end;
+
+(* search from bottom to top, left to right *)
+fun search_bt_valid validf =
+  let
+    fun sf_valid_td_lr z =
+      let val here = if validf z then [Zipper.Here z] else [] in
+        (case Zipper.trm z of
+          _ $ _ =>
+            [Zipper.LookIn (Zipper.move_down_left z),
+             Zipper.LookIn (Zipper.move_down_right z)] @ here
+        | Abs _ => [Zipper.LookIn (Zipper.move_down_abs z)] @ here
+        | _ => here)
+      end;
+  in Zipper.lzy_search sf_valid_td_lr end;
+
+fun searchf_unify_gen f (ctxt, maxidx, z) lhs =
+  Seq.map (clean_unify_z ctxt maxidx lhs) (Zipper.limit_apply f z);
+
+(* search all unifications *)
+val searchf_lr_unify_all = searchf_unify_gen search_lr_all;
+
+(* search only for 'valid' unifiers (non abs subterms and non vars) *)
+val searchf_lr_unify_valid = searchf_unify_gen (search_lr_valid valid_match_start);
+
+val searchf_bt_unify_valid = searchf_unify_gen (search_bt_valid valid_match_start);
+
+(* apply a substitution in the conclusion of the theorem *)
+(* cfvs are certified free var placeholders for goal params *)
+(* conclthm is a theorem of for just the conclusion *)
+(* m is instantiation/match information *)
+(* rule is the equation for substitution *)
+fun apply_subst_in_concl ctxt i st (cfvs, conclthm) rule m =
+  RW_Inst.rw ctxt m rule conclthm
+  |> unfix_frees cfvs
+  |> Conv.fconv_rule Drule.beta_eta_conversion
+  |> (fn r => resolve_tac ctxt [r] i st);
+
+(* substitute within the conclusion of goal i of gth, using a meta
+equation rule. Note that we assume rule has var indicies zero'd *)
+fun prep_concl_subst ctxt i gth =
+  let
+    val th = Thm.incr_indexes 1 gth;
+    val tgt_term = Thm.prop_of th;
+
+    val (fixedbody, fvs) = IsaND.fix_alls_term ctxt i tgt_term;
+    val cfvs = rev (map (Thm.cterm_of ctxt) fvs);
+
+    val conclterm = Logic.strip_imp_concl fixedbody;
+    val conclthm = Thm.trivial (Thm.cterm_of ctxt conclterm);
+    val maxidx = Thm.maxidx_of th;
+    val ft =
+      (Zipper.move_down_right (* ==> *)
+       o Zipper.move_down_left (* Trueprop *)
+       o Zipper.mktop
+       o Thm.prop_of) conclthm
+  in
+    ((cfvs, conclthm), (ctxt, maxidx, ft))
+  end;
+
+(* substitute using an object or meta level equality *)
+fun eqsubst_tac' ctxt searchf instepthm i st =
+  let
+    val (cvfsconclthm, searchinfo) = prep_concl_subst ctxt i st;
+    val stepthms = Seq.of_list (prep_meta_eq ctxt instepthm);
+    fun rewrite_with_thm r =
+      let val (lhs,_) = Logic.dest_equals (Thm.concl_of r) in
+        searchf searchinfo lhs
+        |> Seq.maps (apply_subst_in_concl ctxt i st cvfsconclthm r)
+      end;
+  in stepthms |> Seq.maps rewrite_with_thm end;
+
+
+(* General substitution of multiple occurrences using one of
+   the given theorems *)
+
+fun skip_first_occs_search occ srchf sinfo lhs =
+  (case skipto_skipseq occ (srchf sinfo lhs) of
+    SkipMore _ => Seq.empty
+  | SkipSeq ss => Seq.flat ss);
+
+(* The "occs" argument is a list of integers indicating which occurrence
+w.r.t. the search order, to rewrite. Backtracking will also find later
+occurrences, but all earlier ones are skipped. Thus you can use [0] to
+just find all rewrites. *)
+
+fun eqsubst_tac ctxt occs thms i st =
+  let val nprems = Thm.nprems_of st in
+    if nprems < i then Seq.empty else
+    let
+      val thmseq = Seq.of_list thms;
+      fun apply_occ occ st =
+        thmseq |> Seq.maps (fn r =>
+          eqsubst_tac' ctxt
+            (skip_first_occs_search occ searchf_lr_unify_valid) r
+            (i + (Thm.nprems_of st - nprems)) st);
+      val sorted_occs = Library.sort (rev_order o int_ord) occs;
+    in
+      Seq.maps distinct_subgoals_tac (Seq.EVERY (map apply_occ sorted_occs) st)
+    end
+  end;
+
+
+(* apply a substitution inside assumption j, keeps asm in the same place *)
+fun apply_subst_in_asm ctxt i st rule ((cfvs, j, _, pth),m) =
+  let
+    val st2 = Thm.rotate_rule (j - 1) i st; (* put premice first *)
+    val preelimrule =
+      RW_Inst.rw ctxt m rule pth
+      |> (Seq.hd o prune_params_tac ctxt)
+      |> Thm.permute_prems 0 ~1 (* put old asm first *)
+      |> unfix_frees cfvs (* unfix any global params *)
+      |> Conv.fconv_rule Drule.beta_eta_conversion; (* normal form *)
+  in
+    (* ~j because new asm starts at back, thus we subtract 1 *)
+    Seq.map (Thm.rotate_rule (~ j) (Thm.nprems_of rule + i))
+      (dresolve_tac ctxt [preelimrule] i st2)
+  end;
+
+
+(* prepare to substitute within the j'th premise of subgoal i of gth,
+using a meta-level equation. Note that we assume rule has var indicies
+zero'd. Note that we also assume that premt is the j'th premice of
+subgoal i of gth. Note the repetition of work done for each
+assumption, i.e. this can be made more efficient for search over
+multiple assumptions.  *)
+fun prep_subst_in_asm ctxt i gth j =
+  let
+    val th = Thm.incr_indexes 1 gth;
+    val tgt_term = Thm.prop_of th;
+
+    val (fixedbody, fvs) = IsaND.fix_alls_term ctxt i tgt_term;
+    val cfvs = rev (map (Thm.cterm_of ctxt) fvs);
+
+    val asmt = nth (Logic.strip_imp_prems fixedbody) (j - 1);
+    val asm_nprems = length (Logic.strip_imp_prems asmt);
+
+    val pth = Thm.trivial ((Thm.cterm_of ctxt) asmt);
+    val maxidx = Thm.maxidx_of th;
+
+    val ft =
+      (Zipper.move_down_right (* trueprop *)
+         o Zipper.mktop
+         o Thm.prop_of) pth
+  in ((cfvs, j, asm_nprems, pth), (ctxt, maxidx, ft)) end;
+
+(* prepare subst in every possible assumption *)
+fun prep_subst_in_asms ctxt i gth =
+  map (prep_subst_in_asm ctxt i gth)
+    ((fn l => Library.upto (1, length l))
+      (Logic.prems_of_goal (Thm.prop_of gth) i));
+
+
+(* substitute in an assumption using an object or meta level equality *)
+fun eqsubst_asm_tac' ctxt searchf skipocc instepthm i st =
+  let
+    val asmpreps = prep_subst_in_asms ctxt i st;
+    val stepthms = Seq.of_list (prep_meta_eq ctxt instepthm);
+    fun rewrite_with_thm r =
+      let
+        val (lhs,_) = Logic.dest_equals (Thm.concl_of r);
+        fun occ_search occ [] = Seq.empty
+          | occ_search occ ((asminfo, searchinfo)::moreasms) =
+              (case searchf searchinfo occ lhs of
+                SkipMore i => occ_search i moreasms
+              | SkipSeq ss =>
+                  Seq.append (Seq.map (Library.pair asminfo) (Seq.flat ss))
+                    (occ_search 1 moreasms)) (* find later substs also *)
+      in
+        occ_search skipocc asmpreps |> Seq.maps (apply_subst_in_asm ctxt i st r)
+      end;
+  in stepthms |> Seq.maps rewrite_with_thm end;
+
+
+fun skip_first_asm_occs_search searchf sinfo occ lhs =
+  skipto_skipseq occ (searchf sinfo lhs);
+
+fun eqsubst_asm_tac ctxt occs thms i st =
+  let val nprems = Thm.nprems_of st in
+    if nprems < i then Seq.empty
+    else
+      let
+        val thmseq = Seq.of_list thms;
+        fun apply_occ occ st =
+          thmseq |> Seq.maps (fn r =>
+            eqsubst_asm_tac' ctxt
+              (skip_first_asm_occs_search searchf_lr_unify_valid) occ r
+              (i + (Thm.nprems_of st - nprems)) st);
+        val sorted_occs = Library.sort (rev_order o int_ord) occs;
+      in
+        Seq.maps distinct_subgoals_tac (Seq.EVERY (map apply_occ sorted_occs) st)
+      end
+  end;
+
+(* combination method that takes a flag (true indicates that subst
+   should be done to an assumption, false = apply to the conclusion of
+   the goal) as well as the theorems to use *)
+val _ =
+  let
+    val parser =
+      Scan.lift (Args.mode "asm"
+        -- Scan.optional (Args.parens (Scan.repeat Parse.nat)) [0])
+      -- Attrib.thms
+    fun eqsubst_asm_ctac occs inthms =
+      CONTEXT_TACTIC' (fn ctxt => eqsubst_asm_tac ctxt occs inthms)
+    fun eqsubst_ctac occs inthms =
+      CONTEXT_TACTIC' (fn ctxt => eqsubst_tac ctxt occs inthms)
+  in
+    Theory.setup (
+      Method.setup \<^binding>\<open>sub\<close>
+        (parser >> (fn ((asm, occs), inthms) => fn ctxt => SIMPLE_METHOD' (
+          (if asm then eqsubst_asm_tac else eqsubst_tac) ctxt occs inthms)))
+        "single-step substitution" (* #>
+      Method.setup \<^binding>\<open>subst\<close>
+        (parser >> (fn ((asm, occs), inthms) => K (CONTEXT_METHOD (
+          CHEADGOAL o SIDE_CONDS 0
+            ((if asm then eqsubst_asm_ctac else eqsubst_ctac) occs inthms)))))
+        "single-step substitution with automatic discharge of side conditions" *)
+    )
+  end
+
+end;