[Help-bash] Brace expansion with empty args

[齋藤正樹]

Hi, all.
I have a question about brace expansion.

~$ echo a{b,c}
ab ac
~$ echo a{,c}
a ac
~$ echo a{,}
a a
~$ echo {'',c}
 c
~$

OK, two words were generated.

~$ echo {,c}
c
~$ echo {,}

~$

I expected two words.
But I got only one or zero words.
Is this behavior intended?
Or documented in anywhere?

I tested on 4.2.0(1)-release and 4.1.2(1)-release.

Regards.