Arrolio is a paged-media layout engine: it takes a content tree plus a layout spec (page templates, styles, flows) and produces a sequence of laid-out pages, which a renderer then emits — by default to PDF bytes via the sibling `pdfrb` gem. Arrolio is the middle layer of a three-library stack: * `pdfrb` — pure-Ruby PDF library (bytes <-> model). * `arrolio` — FOP-like paged layout (this gem; depends on pdfrb). * `loom` — multi-target layout compiler (separate gem; emits to Arrolio, CSS, XSL-FO, IDML). Arrolio itself owns: page templates, threaded flows, Knuth-Plass line and page breaking, tables, lists, SVG, page-model selection, running headers/footers, and the rendering pipeline. It does not own the PDF byte format (Pdfrb does) or any DSL (Loom does).