It is possible to stop parsing by raising parse error?
In the financial industry the [ISO 8583](https://en.wikipedia.org/wiki/ISO_8583) protocol is widely used. This protocol consist of:
1. A _bitmap_ -- several bytes, each bit in their determines, that fields present in the message
2. Message fields

For example, first 2 fields of many ISO 8583 implementations can be described by the following `ksy` schema:
```yaml
meta:
  id: iso8583
  endian: be
types:
  str2:
    seq:
      - id: length
        type: u2
      - id: value
        size: length
        type: str
        encoding: ASCII
seq:
  - id: bitmap
    type: b1
    repeat: expr
    repeat-expr: 64
  - id: secondary_bitmap
    type: b1
    repeat: expr
    repeat-expr: 64
    if: bitmap[0]
  - id: pan
    doc: n..19
    type: str2
    if: bitmap[1]
```

Protocol itself is not describes, how to determine field length. So it is not backward-compatible, if you add new field between already defined fields, because old clients will not known, how to skip unknown field -- they do not known its length. That means, that parsers must throws an exception, when it comes with unknown field.

How to express that in `ksy`? Right now, if delete `secondary_bitmap` member from definition and try to parse message, that contains both, `secondary_bitmap` and `pan` fields, `ksy` will parse bytes from `secondary_bitmap`, as `pan` field