bug#11738: 24.1.50; Regression: `modify-frame-parameters' is broken for `left' & `top'

[Drew Adams]

ping.

> emacs -Q
> In *scratch*:
> (setq new (- (frame-parameter nil 'left) 10))
> (setq new `(left + ,new))
> (modify-frame-parameters nil (list new))
>  
> IOW, try to set `left' or `top' to a value that is a cons whose car is
> `left' or `top', whose cadr is `+' or `-', and whose caddr is some new
> value.  Nothing happens.
> 
> This is a regression wrt ALL previous Emacs versions (well, ever
> since we had frames).  It is important that the cons form of these
> parameter values continue to be accepted, as it has always been.