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


DEBUG:Going to execute:
cbv beta