Goal:
  
  A : Type
  t : A
  P : nat → A → A → Type
  i : nat
  hd : A
  tl : list A
  tl' : list A
  X : OnOne2i P (S i) tl tl'
  IHX : ∀ n : nat,
          nth_error tl n = Some t
          → ∑ t' : A,
              nth_error tl' n = Some t' × (t = t') + P (S i + n)%nat t t'
  ============================
   (Some hd = Some t
    → ∑ t' : A, Some hd = Some t' × (t = t') + P (i + 0)%nat t t')


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrrewrite@0> $1 $2