Goal:
  
  G1 : t
  ext : EdgeSet.t
  x : VSet.elt
  e : EdgeSet.elt
  es : list EdgeSet.elt
  H0 : VSet.In x (V G1)
  H1 : VSet.In x (fold_right fold_fun VSet.empty es) ->
       exists e : EdgeSet.elt,
         In e es /\ (x = e..s \/ x = e..t) /\ VSet.In x (V G1)
  H2 : (exists e : EdgeSet.elt,
          In e es /\ (x = e..s \/ x = e..t) /\ VSet.In x (V G1)) ->
       VSet.In x (fold_right fold_fun VSet.empty es)
  H3 : x = e..t
  ============================
   (x = e..s)


DEBUG:Going to execute:
<Coq.Init.Tauto.is_empty> flags X1; elim H
Level 3: Backtrace:

In environment
G1 : t
ext : EdgeSet.t
x : VSet.elt
e : EdgeSet.elt
es : list EdgeSet.elt
H0 : VSet.In x (V G1)
H1 : VSet.In x (fold_right fold_fun VSet.empty es) ->
     exists e : EdgeSet.elt,
       In e es /\ (x = e..s \/ x = e..t) /\ VSet.In x (V G1)
H2 : (exists e : EdgeSet.elt,
        In e es /\ (x = e..s \/ x = e..t) /\ VSet.In x (V G1)) ->
     VSet.In x (fold_right fold_fun VSet.empty es)
H3 : x = e..t
Unable to unify "e..s" with "x".
Level 3: Backtrace:

In environment
G1 : t
ext : EdgeSet.t
x : VSet.elt
e : EdgeSet.elt
es : list EdgeSet.elt
H0 : VSet.In x (V G1)
H1 : VSet.In x (fold_right fold_fun VSet.empty es) ->
     exists e : EdgeSet.elt,
       In e es /\ (x = e..s \/ x = e..t) /\ VSet.In x (V G1)
H2 : (exists e : EdgeSet.elt,
        In e es /\ (x = e..s \/ x = e..t) /\ VSet.In x (V G1)) ->
     VSet.In x (fold_right fold_fun VSet.empty es)
H3 : x = e..t
Unable to unify "e..s" with "x".
Level 4: Backtrace:

In environment
G1 : t
ext : EdgeSet.t
x : VSet.elt
e : EdgeSet.elt
es : list EdgeSet.elt
H0 : VSet.In x (V G1)
H1 : VSet.In x (fold_right fold_fun VSet.empty es) ->
     exists e : EdgeSet.elt,
       In e es /\ (x = e..s \/ x = e..t) /\ VSet.In x (V G1)
H2 : (exists e : EdgeSet.elt,
        In e es /\ (x = e..s \/ x = e..t) /\ VSet.In x (V G1)) ->
     VSet.In x (fold_right fold_fun VSet.empty es)
H3 : x = e..t
Unable to unify "e..s" with "x".
Level 4: Backtrace:

In environment
G1 : t
ext : EdgeSet.t
x : VSet.elt
e : EdgeSet.elt
es : list EdgeSet.elt
H0 : VSet.In x (V G1)
H1 : VSet.In x (fold_right fold_fun VSet.empty es) ->
     exists e : EdgeSet.elt,
       In e es /\ (x = e..s \/ x = e..t) /\ VSet.In x (V G1)
H2 : (exists e : EdgeSet.elt,
        In e es /\ (x = e..s \/ x = e..t) /\ VSet.In x (V G1)) ->
     VSet.In x (fold_right fold_fun VSet.empty es)
H3 : x = e..t
Unable to unify "e..s" with "x".
Level 3: Backtrace:

In environment
G1 : t
ext : EdgeSet.t
x : VSet.elt
e : EdgeSet.elt
es : list EdgeSet.elt
H0 : VSet.In x (V G1)
H1 : VSet.In x (fold_right fold_fun VSet.empty es) ->
     exists e : EdgeSet.elt,
       In e es /\ (x = e..s \/ x = e..t) /\ VSet.In x (V G1)
H2 : (exists e : EdgeSet.elt,
        In e es /\ (x = e..s \/ x = e..t) /\ VSet.In x (V G1)) ->
     VSet.In x (fold_right fold_fun VSet.empty es)
H3 : x = e..t
Unable to unify "e..s" with "x".
Level 3: Backtrace:

In environment
G1 : t
ext : EdgeSet.t
x : VSet.elt
e : EdgeSet.elt
es : list EdgeSet.elt
H0 : VSet.In x (V G1)
H1 : VSet.In x (fold_right fold_fun VSet.empty es) ->
     exists e : EdgeSet.elt,
       In e es /\ (x = e..s \/ x = e..t) /\ VSet.In x (V G1)
H2 : (exists e : EdgeSet.elt,
        In e es /\ (x = e..s \/ x = e..t) /\ VSet.In x (V G1)) ->
     VSet.In x (fold_right fold_fun VSet.empty es)
H3 : x = e..t
Unable to unify "e..s" with "x".