How to check if an array contains a value?
What is the allowable syntax for conditional expressions?

I have to test if a field is equal to any one of 63 values.  The only way I can think to do this is the following, where x1, x2, x3 etc. are integers and a is the field id.

```ksy
if: a==x1 or a==x2 or a==x3 or ... or a==x63
```

There has to be a shorter way.  Maybe something like list inclusion?

```ksy
if: a in [x1, x2, x3, ..., x63]
```
