Stretchy Operators

I'm having a bit of trouble divining from Bruce's proposal just how
stretchy operators are suppose to work.  Consider

  x + 1
( ----- )
  x - 1

As I understand it, the display list for this should look like

(mrow
	(mo "(")
	(mfraction
		(mrow
			(mi "x")
			(mo "+")
			(mn "1")
		)
		(mrow
			(mi "x")
			(mo "-")
			(mn "1")
		)
	)
	(mo ")")		
)

Is the renderer to assume the parentheses grow to match the expression
height?  Or what about an expression like

||x-a| sin(b)|

Typically, I would make the outer vertical rules larger, but I see no
way of creating a display list that would accomplish this.  Am I
missing something?

Robert