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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".
Used the more precise "If the type is nillable, then it expands into an optional type", which is the terminology used in 2.4.3 Content models. - Jerome