The selector element specifies an XPath expression that selects a set of elements for an identity constraint (unique, key, and keyref elements).
(The ? sign declares that the element can occur zero or one time inside the selector element)
|id||Optional. Specifies a unique ID for the element|
|xpath||Required. Specifies an XPath expression, relative to the element being declared, that identifies the child elements to which the identity constraint applies|
|any attributes||Optional. Specifies any other attributes with non-schema namespace|
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