I should be able to use dot in place of arrow whenever it's intuitive

Summary: For example, in my Vacation Photos Space, I tried to say [[Picture._thumbnail]], and it broke cryptically, confusing me for weeks. Of course, [[Picture -> _thumbnail]] worked fine, but if I can get confused by this, it is highly likely that normal users will.

Need to think about this extremely carefully. At the moment, the semantics of dot are very different from those of arrow, but they feel kind of similar. Maybe it should fall back to arrow semantics if that makes sense? If the right-hand-side of the dot is a function or Property, use that?