Goal:
  
  A : Type
  P : A → A → Type
  hd : A
  tl : list A
  tl' : list A
  h : OnOne2 P tl tl'
  x : A
  y : A
  u : list A
  v : list A
  a : P x y
  H : tl = u ++ x :: v
  H0 : tl' = u ++ y :: v
  ============================
   (hd :: tl = (hd :: u) ++ x :: v)


DEBUG:Going to execute:
<Coq.Init.Tauto.is_empty> flags X1; elim H
Level 2: Backtrace:

In environment
A : Type
P : A → A → Type
hd : A
tl, tl' : list A
h : OnOne2 P tl tl'
x, y : A
u, v : list A
a : P x y
H : tl = u ++ x :: v
H0 : tl' = u ++ y :: v
Unable to unify "(hd :: u) ++ x :: v" with "hd :: tl".
Level 2: Backtrace:

In environment
A : Type
P : A → A → Type
hd : A
tl, tl' : list A
h : OnOne2 P tl tl'
x, y : A
u, v : list A
a : P x y
H : tl = u ++ x :: v
H0 : tl' = u ++ y :: v
Unable to unify "(hd :: u) ++ x :: v" with "hd :: tl".
Level 3: Backtrace:

In environment
A : Type
P : A → A → Type
hd : A
tl, tl' : list A
h : OnOne2 P tl tl'
x, y : A
u, v : list A
a : P x y
H : tl = u ++ x :: v
H0 : tl' = u ++ y :: v
Unable to unify "(hd :: u) ++ x :: v" with "hd :: tl".
Level 3: Backtrace:

In environment
A : Type
P : A → A → Type
hd : A
tl, tl' : list A
h : OnOne2 P tl tl'
x, y : A
u, v : list A
a : P x y
H : tl = u ++ x :: v
H0 : tl' = u ++ y :: v
Unable to unify "(hd :: u) ++ x :: v" with "hd :: tl".
Level 2: Backtrace:

In environment
A : Type
P : A → A → Type
hd : A
tl, tl' : list A
h : OnOne2 P tl tl'
x, y : A
u, v : list A
a : P x y
H : tl = u ++ x :: v
H0 : tl' = u ++ y :: v
Unable to unify "(hd :: u) ++ x :: v" with "hd :: tl".
Level 2: Backtrace:

In environment
A : Type
P : A → A → Type
hd : A
tl, tl' : list A
h : OnOne2 P tl tl'
x, y : A
u, v : list A
a : P x y
H : tl = u ++ x :: v
H0 : tl' = u ++ y :: v
Unable to unify "(hd :: u) ++ x :: v" with "hd :: tl".