Where's NSExpression format string syntax documented?

I'm looking for a description of the expression syntax that +[NSExpression expressionWithFormat:] parses, but I can't find anything. There's a whole page on the _predicate_ format string syntax*, but that only covers the syntax specific to predicates and ignores expressions. (For example, it des

AVFoundation error messages when getting frames from video tracks

I've got the following code snippet which is part of code that creates a CGImage. The CGImage is created but the last line of this code snippet triggers the output of an error message to the console. The self._movieAsset is an AVURLAsset created from a local file on disk. I've tried some variati