js.compile.ignore=true
=====
>>> main.whiley
type Msg is {int kind, int[] data}

type Tx is {
  Msg items,
  int index,
  int value,
  Msg result
} where items{data:=items.data[index:=value]} == result

public export method test():
   Msg m = {kind: 0, data:[]}
   Tx t = {items:m,index:0,value:0,result:m}
---
E708 main.whiley 8,31:35
E718 main.whiley 12,10:43
=====
>>> main.whiley 11:12
   Msg m = {kind: 0, data:[1]}
---
E702 main.whiley 12,10:43
E718 main.whiley 12,10:43
=====
>>> main.whiley 12:13
   Tx t = {items:m,index:0,value:0,result:m{data:=[0]}}
---
=====
>>> main.whiley 11:13
   Msg m1 = {kind: 0, data:[1,2]}
   Msg m2 = {kind: 0, data:[1,123]}   
   Tx t = {items:m1,index:1,value:123,result:m2}
---