Sharing parts of documents

I made a to-do list for myself recently, to help me organize a particularly busy day. As I often do, I wrote it as a Google Doc.

I like having a document on the web, since I can then access it from anywhere, and putting everything into a single document makes it easy to move things around, as I organize my thoughts.

I wanted to share parts of this document with some colleagues, but not the whole thing. But Google Docs sets sharing permissions on a per-document basis — not for parts of documents.

It occurs to me that there may not be any tool out there that lets you do that. You can place multiple documents into a folder, giving each document a unique list of “can-edit” or “read-only” collaborators.

But you can’t get that level of control over shareability within a single on-line document. Why hasn’t anyone implemented such a thing yet? Or have they, and I missed it?