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This specification describes the syntax and semantics of XProc: An XML Pipeline Language, a language for describing operations to be performed on XML documents. An XML Pipeline specifies a sequence of operations to be performed on one or more XML documents. Pipelines generally accept one or more XML documents as input and producing one or more XML documents as output, though they are not required to do so. Some pipelines are entirely self-contained, starting with input derived inside the pipeline and producing no XML output.