Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  l2 : list A
  r : list B
  X : All2 R l2 r
  ============================
   (∑ r1 r2 : list B, r = r1 ++ r2 × All2 R [] r1 × All2 R l2 r2)


DEBUG:Going to execute:
<coq-core.plugins.ltac::exists@1> $1