1	import Base
2	
3	# Neutral datatypes shared by src/deque.bend and spec/deque.bend.
4	
5	type Error is Data:
6	  EmptyDeque{}
7	
8	type Op<-T: Data> is Data:
9	  Length{}
10	  PushFront{value: T}
11	  PushBack{value: T}
12	  PopFront{}
13	  PopBack{}
14	  PeekFront{}
15	  PeekBack{}
16	  ToList{}
17	
18	type Obs<-T: Data> is Data:
19	  ONat{value: Nat}
20	  OItem{result: Result<&2, &2, Error, T>}
21	  OUnit{}
22	  OList{items: List<&2, T>}