## Advanced

It is typical to call [#to_enum](http://ruby-doc.org/core-2.3.1/Object.html#method-i-to_enum) when defining methods for a generic Enumerable, in case no block is passed.

Here is an additional test you could add:

```ruby
def test_no_block_is_passed
  skip
  result = [1, 2, 3].accumulate
  assert_instance_of Enumerator, result
end
```