Make find-persistent-objects behave identically across different stores

[html]

Currently find-persistent-objects has similar key params like :filter :filter-fn, :where

memory - filter
prevalence - filter
elephant - filter-fn
postmodern - where
clsql - where

I suppose it needs at least to use filter key for elephant.
To make single interface for all stores it needs to make some abstract sub language.
I have such language in git://github.com/html/weblocks-filtering-widget.git but it is not ready to use.