Summing the values in a sequence doesn't work
There is no built-in way to sum values, so I have tried a nasty hack
```yaml
types:
  field:
    seq:
      - id: distance
        type: u2
      - id: elevation_count
        type: u1
      - id: azimuth_counts
        type: azimuth(_index)
        repeat: expr
        repeat-expr: elevation_count
    types:
      azimuth:
        params:
          - id: index
            type: u1
        seq:
          - id: value
            type: u1
        instances:
          sum:
            value: (index>0?_parent.azimuth_counts[index-1].sum:0)+value
```

It doesn't work (in webide). If I replaced ```sum``` by ```value``` in the last line, it parses. It seems it is somehow related to laziness.