how do I use the {.this.} pragma?

[cy]

I'm not sure how to use the new {.this.} pragma, that lets us use shortcutted 
syntax that adds in an implicit "this" when possible.
    
    
    type Bar = object
        baz: int
    
    {.this: self.}
    proc foo(self: Bar):
      echo(baz)
    
    foo(Bar(42))
    

> nim --experimental c thing.nim

thing.nim(6, 8) Error: undeclared identifier: 'baz'

> nim --version
    
    
    Nim Compiler Version 0.14.3 (2016-08-09) [Linux: amd64]
    Copyright (c) 2006-2016 by Andreas Rumpf
    
    git hash: 5d05ee21c217e3585bf2eec9158e7aff8236894b
    active boot switches: -d:release