I can easily edit a part of a nicely rendered display

Summary: Eric requested something along these lines, but I've been thinking about it for a long time.

I think the notion is the _editable function. This receives any value, and a couple of QL expressions. One expression is how to display this value; the other is how to edit the value. If you double-click on the display (or something like that), it swaps in the specified editor, with some sort of Done button. When you hit Done, it recalculates and puts back the display expression.

Both expressions would default -- the default render is however this value normally renders, and the default edit render is whatever would normally come out of _edit.

There needs to be an affordance that says, "this can be edited". Not sure what that should be -- possibly a slight background color, or a border, or an icon, or something like that. This also might be optional -- possibly you can specify a style, and there is a default style for this?