Github user cestella commented on the issue:
https://github.com/apache/metron/pull/1021
@ottobackwards As long as the underlying map is updated in
`MapVariableResolver` as per the `assign` function in that variable resolver,
it should be supported without any change as the approach is to return the
ConcatMap, which is backed by the real maps, which are updated. ConcatMap is
immutable, but the underlying maps that backs it aren't and assign would update
those maps directly.---
