I think my understanding of the meaning of the term "nesting" might differ from others'. The term has a well-defined, formal meaning in IT. It means one object fully contained within another object (or nested inside another):

1-2-2-1 is nesting.
1-2-3-3-2-1 is 2 level nesting.

1-2-1-2 is not nesting (because there's an ovelap).

What YOWkid is describing (if I understand him correctly) is nesting -- not b-to-b.

Maybe the term is used differently in the context of airline tickets.