When one exchange region is completely contained within another,
replace the larger one with the smaller one.
The term "expand" may seem counter-intuitive, as the result is less
text, but think of it this way: the smaller region is "expanding"
to occupy the space previously defined by the larger region.
Any custom mappings to <Plug>Exchange, <Plug>ExchangeLine, or
<Plug>ExchangeClear will have to be changed to map to <Plug>(Exchange),
<Plug>(ExchangeLine), and <Plug>(ExchangeClear).
References #12
