Found in chooser output, could well exist elsewhere.

Why?

It is good that we're using DC Terms, which supercedes DC Elements, but dc: is
the best namespace prefix to use. It is baked into RDFa 1.1
http://www.w3.org/2011/rdfa-context/rdfa-1.1.html

If xmlns gets stripped off, dc: will mean the right thing, that is
http://purl.org/dc/terms/ while dct: will mean nothing.