Goal:
  
  A : Type
  B : Type
  P : B → B → Type
  Q : A → B → Type
  R : A → B → Type
  H' : All2 Q [] []
  H'' : All2 R [] []
  ============================
   (∑ l''' : list B, All2 P [] l''' × All2 P [] l''')


DEBUG:Going to execute:
exists []