## 3.1.4

- sort, uniq, group and sort_uniq combinators implemented
- some conversion functions that use Sequence.t2
- infix operators in Sequence.Infix
- Sequence.t2 type for efficient iteration on pairs of elements
- some combinators are adapted to Sequence.t2
- zip, unzip and zip_i to convert between t and t2
- added scan combinator