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.Note that the concepts of a fixed value and a default value are mutually exclusive, and so it is an error for a declaration to contain both fixed and default attributes.The values of the attributes used in element and attribute declarations to constrain their occurrences are summarized in.Table 1.Occurrence Constraints for Elements and AttributesElements(, ) , Attributes, ,Notes(1, 1) -, -required, -, -element/attribute must appear once, it may have any value(1, 1) 37, -required, 37, -element/attribute must appear once, its value must be 37(2, unbounded) 37, -n/aelement must appear twice or more, its value must be 37; in general, and values may be positive integers, and value may also be "unbounded"(0, 1) -, -optional, -, -element/attribute may appear once, it may have any value(0, 1) 37, -optional, 37, -element/attribute may appear once, if it does appear its value must be 37, if it does not appear its value is 37(0, 1) -, 37optional, -, 37element/attribute may appear once; if it does not appear its value is 37, otherwise its value is that given(0, 2) -, 37n/aelement may appear once, twice, or not at all; if the element does not appear it is not provided; if it does appear and it is empty, its value is 37; otherwise its value is that given; in general, and values may be positive integers, and
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