List of all built-in variables
I notice that KSC recognises that list of variables in expressions, but not all of them described neigher in [User Guide](https://doc.kaitai.io/user_guide.html) or [KSY Reference](https://doc.kaitai.io/ksy_reference.html). Because I implementing [alternative compiler](https://github.com/Mingun/ksc-rs) I need to known which variables availible and in which contexts.

I found, that KSC has following list of variables (https://github.com/kaitai-io/kaitai_struct_compiler/blob/eb420a617ae91a803659dfc29c8a8b54516b7b70/shared/src/main/scala/io/kaitai/struct/format/Identifier.scala#L77-L86)
  * `_root`
  * `_parent`
  * `_io`
  * `_`
  * `_buf`
  * `_index`
  * `_on`
  * `_is_le`
  * `_sizeof`

Can someone helps me to fill the following table:

|Variable|Context|Meaning|
|--------|-------|-------|
|`_root` |`instances.<...>.io`|Reference to root type in the current file|
|...|...|...|

(*Context* -- name of KSY property in which expression can be used)