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Many of the elements that make up a mapping rule need some kind of information as input. The xfMap provides a general input mechanism with expression elements. We can therefore start to discuss these expression elements regardless of the xfMap elements that may use them.
Expression elements are so called because they contain an expr attribute whose value we call the expression string. This section describes two types of expression elements: the extract and literal expressions. Other types of expression elements are described in later sections.