Some time ago, Hrvoje Niksic wrote...
|+
| Kirill, don't you think it would be better to have the placement
| specifiers named `vertical-scrollbar-placement' (accepting `left' and
| `right') and `horizontal-scrollbar-placement' (accepting `top' and
| `bottom')? This would be closer to their resources equivalents,
| analogous to -visible-p variants, and probably more intuitive.
This requred special kind of specifier, like (make-specifier 'left-right).
Generic specifiers do not play tricks here because when you assign
'bottom to vertical scrollbar specifier, the scrollbar goes either
on left or right (it has nowhere else to go), but specifier instance is
still 'bottom, which is very confusing.
I think that left and right better fit into customize options. For one
who understand specifications, abstraction of left-p set to nil being
"right" ans set to t being "left" is very simple.
Kirill