1	import Base
2	import ./owner_product.bend as Product
3	# Only canonical product exchanges create these inverses. No body-provided inverse.
4	def replaced(~State:Type,~P:Type,~Rest:Type,~lens:Product.Lens<State,P,Rest>,replacement:P,taken:Rest & P) -> State & P:
5	  match taken:
6	    case (rest,old):(Product.put_field(~State,~P,~Rest,lens)(rest,replacement),old)
7	def exchange(~State:Type,~P:Type,~Rest:Type,~lens:Product.Lens<State,P,Rest>,state:State,replacement:P) -> State & P:
8	  replaced(~State,~P,~Rest,~lens,replacement,Product.get_field(~State,~P,~Rest,lens)(state))
9	def appended(~Recovery:Type,~P:Type,~Rest:Type,~lens:Product.Lens<Recovery,List<P>,Rest>,value:P,taken:Rest & List<P>) -> Recovery:
10	  match taken:
11	    case (rest,values):Product.put_field(~Recovery,~List<P>,~Rest,lens)(rest,value <> values)
12	def append(~Recovery:Type,~P:Type,~Rest:Type,~lens:Product.Lens<Recovery,List<P>,Rest>,recovery:Recovery,value:P) -> Recovery:
13	  appended(~Recovery,~P,~Rest,~lens,value,Product.get_field(~Recovery,~List<P>,~Rest,lens)(recovery))