The key element specifies an attribute or element value as a key (unique, non-nullable, and always present) within the containing element in an instance document.
The key element MUST contain the following (in order):
(The ? sign declares that the element can occur zero or one time, and the + sign declares that the element must occur one or more times inside the key element)
|id||Optional. Specifies a unique ID for the element|
|name||Required. Specifies the name of the key element|
|any attributes||Optional. Specifies any other attributes with non-schema namespace|
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