Show/Don't show lines based on pattern...

It seems so useful, I have to believe it's out there, but called by some
name I haven't thought of...

I'd like to show only the lines that match (or don't match) a particular
regular expression.

Once I am looking at that subset of the lines, I'd like to either exclude
(or include) more lines,

Or expand one particular `hidden' block.

So, sample file:

1a
1b
2a
2b
2c
3a
3c

Selective display only lines with /a/:
1a
2a
3a

Additionally, suppress lines that contain /2/:
1a
3a

Now, show the whole block suppressed under /3a/:
1a
3a
3c

`list-matching-lines' doesn't quite cover the requirement.

It seems like all the components are there (though I don't know how to use
them, yet):
set-selective-display,
put-text-property,
overlays.
I just can't believe someone hasn't done this already...

Much thanks in advance.
Matthew Miner
(BTW, I am using GNU emacs, if that's relevant. -- is there a separate
group for that?)


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