The XProc 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 zero or more XML documents. Pipelines generally accept zero or
more XML documents as input and produce zero or more XML documents
as output. Pipelines are made up of simple steps which perform
atomic operations on XML documents and constructs similar to
conditionals, iteration, and exception handlers which control which
steps are executed.