If no other modifiers are defined on the Sequence modifier, each subelement in this element must appear exactly once. In other words, it is required. Also, the subelements must appear in the order in which they are specified in the referencing element. You can define other modifiers on the Sequence modifier. In this way, you can specify that some subelements are optional, some appear zero or more times, and some appear one or more times.
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