Compiles incorrect rules – referencing "this" inside helper function

```
type InitialTimestamp extends Number {
  validate() { initial(now) }
}

initial(init) { prior(this) == null ? this == init : this == prior(this) }
```

compiles to

```
".validate": "(newData.isNumber() && (this == null ? this == now : this == this))"
```

Would be nice if this just threw an error at compile time