preserve 0.8.1's behavior for correlation when no correlate options are given, not
to mention 0.7 and prior's behavior of not propagating implicit correlation more than
one level..
this is to reduce surprises/hard-to-debug situations when a user isn't trying
to correlate anything.
- more docs + changelog
don't need this...
implicit correlation does not need to consider the explicit_correlate_froms collection