function test_rfind()
  check_eq(rfind('abc', 'c'), 3, 'final char')
  check_eq(rfind('acbc', 'c', 3), 2, 'previous char')
  check_nil(rfind('abc', 'd'), 'missing char')
  check_nil(rfind('abc', 'c', 2), 'no more char')
end