So I am trying to migrate stuff from 0.2 to 0.4 and I am not sure how this worked before.  If I give input/output format as RCFile but provide a storage driver that assumed a different format, how would this work?  Seems like this worked before in 0.2 but I never touched drivers so not sure what happened in the past or what this syntax meant. 

Any information would be helpful.  Thanks!