>
>> If you take the algorithm, it would start on the "u", extend to the 
>> place it extends on the picture on the right, and on the next lines two 
>> whitespace characters on the left would not be displayed as green 
>> anymore (that is, the green area would start under the "f" of "(if".
>
> Why is that? That starting position of the second line's highlight 
> doesn't correspond to anything "natural", does it?  I would expect 
> either that it starts as it does now, directly under the `(' of the 
> first line, or directly under the `u' that started the overlay.
>

In fact it does, the overlay goes upwards above the comment below the "f" 
of "(if".  I attach a new picture between two arbitrary points that are 
not brackets.  I don't see any other way to define in simple terms how the 
whitespace on the left should be trimmed.  The "visualblock" would thus 
highlight all the text of the block between overlay-begin and overlay-end, 
and would be a rectangle except for the first and last lines, that is, 
except before overlay-begin and after overlay-end.

>
> In any case, overlays, and highlighting, are not just for code.
>

Indeed.  I think the proposed "visualblock" attribute is general and 
simple enough that it could be used not only for code.  E.g. it can be 
used to highlight a portion of text with whitespace on the left, a TODO 
item, say.

Gregory