Goal:
  
  A : Type
  B : Type
  C : Type
  R : nat → A → C → Type
  g : A → B → C
  k : nat
  X : All2i (λ (i : nat) (x : A) (y : B), R i x (g x y)) k [] []
  ============================
   (All2i R k [] [])


DEBUG:Going to execute:
do
![ solve
 [ trivial | apply : eq_sym ; trivial ]
 | discriminate
 | contradiction
 | Ltac.split ]