Ada reference manual rule for doubled parentheses around if_expression?

[Stephen Leake <stephen_leake@stephe-leake.org>]

A question recently came up on the Emacs ada-mode list; my Emacs
ada-mode parser rejects this if_expression in an association list:

   Tmp : Boolean := Id (if True then True else True);

but accepts this:

   Tmp : Boolean := Id ((if True then True else True));

I remember seeing some rule in the Ada reference manual that says the
doubled parens are not needed here. But now I can't find the rule, and I
don't want to edit my grammar without a reference.

Anyone know where that rule is? or what to search for?

-- 
-- Stephe