It's not. The same way master^0^0~4 is not useful, yet it's works;
it's a logical result from the syntax.
It's logical if you explain @ as a shortcut for @{0}, and make
@{0}@{1} resolve somehow [*1*].
[Footnotes]
*1*: The best I can do is @{0} is like a ref (while @{>0} are
revisions), but you can't update-ref or symbolic-ref it. Kludgy,
don't you think?