GCC macro with variable number of argument

#define pr(a, b...) printf(a, b) /* (*A) */
#define pr(a, b...) printf(a, ##b) /* (*B) */

pr("Hello\n"); /* (*1) */
pr("Hello %s\n", "World"); /* (*2) */ 
pr("Hello %s %s\n", "My", "World"); /* (*3) */

Above two kinds of macros looks like same at first look. But, there is difference.
In case of (*2) and (*3), (*A) and (*B) both work well.

In terms of (*1), it doesn't have second arguement. That is, it doesn't have arugment 'b'.
So, let's guess result of preprocessing.
Logically, in both case - (*A) and (*B) - "printf(a, )" should be a result, and GCC should complain about this syntax.
But, actual result is, (*A) doesn't work, but (*B) works well.

I didn't check GCC Spec. for this case but, it's interesting enough to leave post :-).
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