Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  x : A
  y0 : B
  l' : list B
  r : R x y0
  a : All2 R [] l'
  ============================
   (let H := CoreTactics.block in ∑ y' : B, y0 :: l' = [y'] × R x y')


DEBUG:Going to execute:
tac H