Goal:
  
  A : Type
  H : ReflectEq A
  a1 : A
  l1 : list A
  a2 : A
  l2 : list A
  Heq : (a1 == a2) = false
  ============================
   (([], a1 :: l1, a2 :: l2) = split_prefix (a1 :: l1) (a2 :: l2)
    → let H := CoreTactics.block in
      let
      '(prefix, l1', l2') := split_prefix (a1 :: l1) (a2 :: l2) in
       a1 :: l1 = prefix ++ l1' ∧ a2 :: l2 = prefix ++ l2')


DEBUG:Going to execute:
cbv beta