Goal:
  
  A : Type
  H : ReflectEq A
  a1 : A
  l1 : list A
  a2 : A
  l2 : list A
  prefix : list A
  l1' : list A
  l2' : list A
  Heq : split_prefix l1 l2 = (prefix, l1', l2')
  Heq0 : (a1 == a2) = true
  Heqcall : (a1 :: prefix, l1', l2') = split_prefix (a1 :: l1) (a2 :: l2)
  H0 : l1 = prefix ++ l1'
  H1 : l2 = prefix ++ l2'
  ============================
   (a1 :: l1 = (a1 :: prefix) ++ l1')


DEBUG:Going to execute:
t; easy