1	import Base
2	
3	# Neutral datatypes shared by src/stack.bend and spec/stack.bend.
4	
5	type Error is Data:
6	  EmptyStack{}
7	
8	type Op<-T: Data> is Data:
9	  Length{}
10	  Push{value: T}
11	  Pop{}
12	  Peek{}
13	  ToList{}
14	
15	type Obs<-T: Data> is Data:
16	  ONat{value: Nat}
17	  OItem{result: Result<&2, &2, Error, T>}
18	  OUnit{}
19	  OList{items: List<&2, T>}