Goal:
  
  elt : Type
  eq_elt : elt -> elt -> Prop
  m : t elt
  m' : t elt
  H : forall x y : key, E.eq x y -> find x m = find y m
  H0 : forall x y : key, E.eq x y -> find x m' = find y m'
  H1 : forall x : key × elt,
       List.In x (elements m) ->
       let
       '(k, _) := x in
        match find k m with
        | Some x0 =>
            match find k m' with
            | Some y0 => eq_elt x0 y0
            | None => False
            end
        | None => match find k m' with
                  | Some _ => False
                  | None => True
                  end
        end
  H3 : forall (k : key) (e : elt),
       List.In (k, e) (elements m') ->
       let
       '(k0, _) := (k, e) in
        match find k0 m with
        | Some x =>
            match find k0 m' with
            | Some y0 => eq_elt x y0
            | None => False
            end
        | None => match find k0 m' with
                  | Some _ => False
                  | None => True
                  end
        end
  k : key
  k0 : key
  e : elt
  H2 : E.eq k (k0, e).1
  H4 : List.In (k0, e) (elements m')
  ============================
   ?T0


DEBUG:Going to execute:
let H := open_constr:((H _)) in
           guarded_specialize_term_under_binders_by' guard_tac H tac
DEBUG LTAC Evaluated term: pair
Level 0: Backtrace:

In environment
elt : Type
eq_elt : elt -> elt -> Prop
m, m' : t elt
H : forall x y : key, E.eq x y -> find x m = find y m
H0 : forall x y : key, E.eq x y -> find x m' = find y m'
H1 : forall x : key × elt,
     List.In x (elements m) ->
     let
     '(k, _) := x in
      match find k m with
      | Some x0 =>
          match find k m' with
          | Some y0 => eq_elt x0 y0
          | None => False
          end
      | None => match find k m' with
                | Some _ => False
                | None => True
                end
      end
H3 : forall (k : key) (e : elt),
     List.In (k, e) (elements m') ->
     let
     '(k0, _) := (k, e) in
      match find k0 m with
      | Some x =>
          match find k0 m' with
          | Some y0 => eq_elt x y0
          | None => False
          end
      | None => match find k0 m' with
                | Some _ => False
                | None => True
                end
      end
k, k0 : key
e : elt
H2 : E.eq k (k0, e).1
H4 : List.In (k0, e) (elements m')
Unable to unify "?A × ?B" with
 "(fix In (a : key × elt) (l : list (key × elt)) {struct l} : Prop :=
     match l with
     | [] => False
     | b :: m => b = a \/ In a m
     end) (?k, ?e) (elements m')".
Level 13: Backtrace:

In environment
elt : Type
eq_elt : elt -> elt -> Prop
m, m' : t elt
H : forall x y : key, E.eq x y -> find x m = find y m
H0 : forall x y : key, E.eq x y -> find x m' = find y m'
H1 : forall x : key × elt,
     List.In x (elements m) ->
     let
     '(k, _) := x in
      match find k m with
      | Some x0 =>
          match find k m' with
          | Some y0 => eq_elt x0 y0
          | None => False
          end
      | None => match find k m' with
                | Some _ => False
                | None => True
                end
      end
H3 : forall (k : key) (e : elt),
     List.In (k, e) (elements m') ->
     let
     '(k0, _) := (k, e) in
      match find k0 m with
      | Some x =>
          match find k0 m' with
          | Some y0 => eq_elt x y0
          | None => False
          end
      | None => match find k0 m' with
                | Some _ => False
                | None => True
                end
      end
k, k0 : key
e : elt
H2 : E.eq k (k0, e).1
H4 : List.In (k0, e) (elements m')
Unable to unify "?A × ?B" with
 "(fix In (a : key × elt) (l : list (key × elt)) {struct l} : Prop :=
     match l with
     | [] => False
     | b :: m => b = a \/ In a m
     end) (?k, ?e) (elements m')".
Level 15: Backtrace:

In environment
elt : Type
eq_elt : elt -> elt -> Prop
m, m' : t elt
H : forall x y : key, E.eq x y -> find x m = find y m
H0 : forall x y : key, E.eq x y -> find x m' = find y m'
H1 : forall x : key × elt,
     List.In x (elements m) ->
     let
     '(k, _) := x in
      match find k m with
      | Some x0 =>
          match find k m' with
          | Some y0 => eq_elt x0 y0
          | None => False
          end
      | None => match find k m' with
                | Some _ => False
                | None => True
                end
      end
H3 : forall (k : key) (e : elt),
     List.In (k, e) (elements m') ->
     let
     '(k0, _) := (k, e) in
      match find k0 m with
      | Some x =>
          match find k0 m' with
          | Some y0 => eq_elt x y0
          | None => False
          end
      | None => match find k0 m' with
                | Some _ => False
                | None => True
                end
      end
k, k0 : key
e : elt
H2 : E.eq k (k0, e).1
H4 : List.In (k0, e) (elements m')
Unable to unify "?A × ?B" with
 "(fix In (a : key × elt) (l : list (key × elt)) {struct l} : Prop :=
     match l with
     | [] => False
     | b :: m => b = a \/ In a m
     end) (?k, ?e) (elements m')".
Level 14: Backtrace:

In environment
elt : Type
eq_elt : elt -> elt -> Prop
m, m' : t elt
H : forall x y : key, E.eq x y -> find x m = find y m
H0 : forall x y : key, E.eq x y -> find x m' = find y m'
H1 : forall x : key × elt,
     List.In x (elements m) ->
     let
     '(k, _) := x in
      match find k m with
      | Some x0 =>
          match find k m' with
          | Some y0 => eq_elt x0 y0
          | None => False
          end
      | None => match find k m' with
                | Some _ => False
                | None => True
                end
      end
H3 : forall (k : key) (e : elt),
     List.In (k, e) (elements m') ->
     let
     '(k0, _) := (k, e) in
      match find k0 m with
      | Some x =>
          match find k0 m' with
          | Some y0 => eq_elt x y0
          | None => False
          end
      | None => match find k0 m' with
                | Some _ => False
                | None => True
                end
      end
k, k0 : key
e : elt
H2 : E.eq k (k0, e).1
H4 : List.In (k0, e) (elements m')
Unable to unify "?A × ?B" with
 "(fix In (a : key × elt) (l : list (key × elt)) {struct l} : Prop :=
     match l with
     | [] => False
     | b :: m => b = a \/ In a m
     end) (?k, ?e) (elements m')".
Level 13: Backtrace:

In environment
elt : Type
eq_elt : elt -> elt -> Prop
m, m' : t elt
H : forall x y : key, E.eq x y -> find x m = find y m
H0 : forall x y : key, E.eq x y -> find x m' = find y m'
H1 : forall x : key × elt,
     List.In x (elements m) ->
     let
     '(k, _) := x in
      match find k m with
      | Some x0 =>
          match find k m' with
          | Some y0 => eq_elt x0 y0
          | None => False
          end
      | None => match find k m' with
                | Some _ => False
                | None => True
                end
      end
H3 : forall (k : key) (e : elt),
     List.In (k, e) (elements m') ->
     let
     '(k0, _) := (k, e) in
      match find k0 m with
      | Some x =>
          match find k0 m' with
          | Some y0 => eq_elt x y0
          | None => False
          end
      | None => match find k0 m' with
                | Some _ => False
                | None => True
                end
      end
k, k0 : key
e : elt
H2 : E.eq k (k0, e).1
H4 : List.In (k0, e) (elements m')
Unable to unify "?A × ?B" with
 "(fix In (a : key × elt) (l : list (key × elt)) {struct l} : Prop :=
     match l with
     | [] => False
     | b :: m => b = a \/ In a m
     end) (?k, ?e) (elements m')".
Level 12: Backtrace:

In environment
elt : Type
eq_elt : elt -> elt -> Prop
m, m' : t elt
H : forall x y : key, E.eq x y -> find x m = find y m
H0 : forall x y : key, E.eq x y -> find x m' = find y m'
H1 : forall x : key × elt,
     List.In x (elements m) ->
     let
     '(k, _) := x in
      match find k m with
      | Some x0 =>
          match find k m' with
          | Some y0 => eq_elt x0 y0
          | None => False
          end
      | None => match find k m' with
                | Some _ => False
                | None => True
                end
      end
H3 : forall (k : key) (e : elt),
     List.In (k, e) (elements m') ->
     let
     '(k0, _) := (k, e) in
      match find k0 m with
      | Some x =>
          match find k0 m' with
          | Some y0 => eq_elt x y0
          | None => False
          end
      | None => match find k0 m' with
                | Some _ => False
                | None => True
                end
      end
k, k0 : key
e : elt
H2 : E.eq k (k0, e).1
H4 : List.In (k0, e) (elements m')
Unable to unify "?A × ?B" with
 "(fix In (a : key × elt) (l : list (key × elt)) {struct l} : Prop :=
     match l with
     | [] => False
     | b :: m => b = a \/ In a m
     end) (?k, ?e) (elements m')".