I think the problem is, the index context needs to be a higher priority than the new context element, but when referencing data with `{.}` the new context element needs to take priority. I could either combine entries in the context tree so that variables like $idx and $len live side-by-side with real context elements, or try to implement a way to skip over index context elements when walking up the tree.
Turns out explicit context setting works in a lot more places than the official dustjs pages indicate. Also some things don't make much sense (like setting it on a block works but not on an inline partial).
