What the Hell is a "self-licking ice-cream cone"?

I'm not really sure about that one, but it has something to do with reclassifying declassified documents. I'm not talking about moving them to another section of the library, say from young-adult to non-fiction. I'm talking about denying they ever existed. 50 year old documents that have been publicly available and republished in some cases, all of a sudden too sensitive to be read? Even the project responsible for this task is classified, the only reason someone noticed is because the documents he had read before were disappearing around him. I tell ya, there's something about a government that does things it can't tell anyone about that unsettles me. 'I can't tell you that I'm covering up that I'm covering something up.' Hmmm... maybe that's a self-licking ice cream cone.

Defense Tech: Decades-Old Docs Reclassified