How to point to a separate data spot in a sequence Kaitai? 
I want to build a feature in the .ksy file to reference data from a later data set to satisfy an earlier condition. In the example I included below, it will produce a value of **null** for the x and y fields. The reason is that the condition value is null since the z field comes after the x and y fields. Is there a method that would allow me to bypass this issue? 
```ksy
seq:
 - id: x
   type: b2
   if: z == false
 - id: y1
   type: b1
   if:  z == true
 - id: y1
   type: b1
   if:  z == true
 - id: z
   type: b1
   enum: z_kind
enum:
  z_kind:
  0: false
  1: true
```
