Clarify meaning of Nullable for collection-valued structural properties

$Nullable The value of $Nullable is one of the Boolean literals true or false. Absence of the member means false. For single-valued properties the value true means that the property allows the null value. For collection-valued properties the property value will always be a collection that MAY be empty. In this case $Nullable applies to items of the collection and specifies whether the collection MAY contain null values.