use must for positive expectations and wont for negative expectations.
- must_be | list.size.must_be :==, 0
- must_be_close_to | subject.size.must_be_close_to 1,1
- must_be_empty | list.must_be_empty
- must_be_instance_of | list.must_be_instance_of Array
- must_be_kind_of | list.must_be_kind_of Enumerable