2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/73233This article was originally published in Topology 22 (1983). The present hyperTeXed redaction includes references to post-1983 results as Addenda, and corrects a few typographical errors. (See math.GT/0411115 for a more comprehensive overview of the subject as it appears 21 years later.)12 pages, 2 figuresGeometric Topology57M25text