How to make hash map instance in kaitai struct?
I want to make a byte code parser in kaitai struct. In [the .pyc file format](http://formats.kaitai.io/python_pyc_27/index.html), bytecodes are defined in enum, but to determine number of arguments for each byte code,  the .pyc file format still uses conditional expression.

So I think, is there anyway to make a hash map that can define both bytecode name and number of bytecode arguments, something like this (I take the below from .pyc file format, but I want to use it for another format):
```yaml
type:
  instruction:
      seq:
        - id: op_code # assume this is always correct
          type: u1
        - id: args
          type: argType(op_code)
          repeat: expr
          repeat-expr: op_codes[op_code][1] # the important line is here
instances:
  op_codes:
    value: # how to realize this?
      0 : ['stop_code', 1]  #[<op_code name>, <op_code_argc>]
      1 : ['pop_top', 2]
      2 : ['rot_two', 3]
      3 : ['rot_three', 4]
```