2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/147746We investigate the causal structure of spacetimes $(M, g)$ for which the metric $g$ is singular on a set of points.uuencoded file containing slightly revised and improved version of 20 pages LaTeX with four figures, expanded version of a talk presented at the 6th Canadian Conference on General Relativity and Relativistic AstrophysicsGeneral Relativity and Quantum Cosmologytext