The principle of Indirection.
In a lump file, the hyper layer is incorporated into the content layer. In HTML, you may set some text as a title with the <h1>
tag, and you may ancar some text to somewere else with an <a>
tag, but the former is content level, and the latter is hyper level. How in this scheme can you have backlinks? These kinds of connections are nesseseraly one way.
If, however we seperate out the hyper level information from the content level information we can enable two way links. In xanadu the document has a list of content, (CONTENT PORTION ADDRESS)
called an Edit Decision List (EDL). This list is loaded, then the portions are sent for. Thus document structure and its content are uncoupled, identical content can appear in multiple documents, and thus the two way link semantics is at hand.