"in1" attribute for feFlood element

In SVG 1.1 and 1.2, feFlood has an IDL attribute "in1". But feFlood doesn't
take any filter image as input, so what is it for? Is this just a mistake?

Rob
-- 
"He was pierced for our transgressions, he was crushed for our iniquities;
the punishment that brought us peace was upon him, and by his wounds we are
healed. We all, like sheep, have gone astray, each of us has turned to his
own way; and the LORD has laid on him the iniquity of us all." [Isaiah
53:5-6]