Book excerpt: XProc 3.0 is a programming language for processing XML, JSON, and other documents in pipelines. XProc chains conversions and other steps, allowing for potentially complex processing. XProc is especially useful for applications, such as publishing, where content may come from multiple input sources, pass through multiple processing steps and result in multiple output streams. XProc 3.0 Programmer Reference is aimed at programmers and others who process XML. It explains the language in detail, provides examples, and contains a set of example use cases. Anyone who uses the XProc language will find a wealth of information in this book.