Github user cestella commented on the issue:
https://github.com/apache/metron/pull/1021
@ottobackwards I don't see it really affecting assignment. `_` should
behave just like any other variable to my mind and return a map. For instance,
if you say `new_field := _`, then I'd expect `new_field` to be a map with all
of the existing fields for a message. We should prevent `_` from being on the
left-hand-side, though, I think.---
