8.1.9 Type expansion
The sentence prior to the first inference says "If the type is 
nillable, then its expansion is optional".  Normally, the word
"optional" means "you have a choice whether to do this 
(ie, the type expansion) or not".  But you do not mean that it
is implementation-defined or implementation-dependent whether to
expand a nillable type.  What you mean is that the result of the
expansion includes the "?" quantifier.  It would be better to 
reword this sentence, avoiding the word "optional". 
For example, "...then the quantifier of the type is zero-or-one".