- lots of docs
- however i might remove the "froms" argument...
- based on the insight that a SELECT can correlate anything that ultimately
came from an enclosing SELECT that links to this one via WHERE/columns/HAVING/ORDER BY,
have the compiler keep track of the FROM lists that correspond in this way,
link it to the asfrom flag, so that we send to _get_display_froms() the exact
list of candidate FROMs to correlate.  no longer need any asfrom logic in the
Select() itself and the whole "froms" argument goes away, the test cases in