extensively opens to enhancement all aspects of a digital document system. Document content is constructed from layers of often heterogeneous type, each with specialized purpose, all semantically aligned. All user-visible document functionality is constructed from stylized program components called behaviors. Document system operations, such as drawing a representation of the document on the screen and saving an edited version, derive from the fundamental operation found to some degree in every digital document system, newly codified as extensible programmatic protocols. This diverse open content, open functionality, and open operation are woven together by numerous mechanisms to produce a final composition that appears built from the ground up as a unified whole."