What does @{FOOBAR@a} mean? (bash)

Observe:

$ unset FOOBAR
$ echo ${FOOBAR@a}

$ FOOBAR=
$ echo ${FOOBAR@a}

$ export FOOBAR
$ echo ${FOOBAR@a}
x
$

It seems that it prints "x" iff the variable has been exported.
This works with any variable - the output is always "x" (iff it is exported).

I can find no mention of this feature in "man bash".  Note: I am not saying
it isn't in there - just that I couldn't find it.

Note that any other letter (such as "b") generates an error message if
FOOBAR is non-empty:

$ echo ${FOOBAR@b}

$ FOOBAR=Test
$ echo ${FOOBAR@b}
bash: ${FOOBAR@b}: bad substitution
$

What is going on?

--

No, I haven't, that's why I'm asking questions. If you won't help me,
why don't you just go find your lost manhood elsewhere.

CLC in a nutshell.