Relevance-ranked output and hypertext were considered by Hildreth to be promising in 1997.43 OPACs have not been conceived within a true hypertext environment, but rather they maintain the structure of their original formats, principally machine-readable cataloging (MARC), and therefore impede the generation of a structure of nodes and links. In addition to continuing to employ MARC format as its underlying structure, the concept of main entry and added entry, field label, and display logic all reflect cataloging rules