Many different and complementary strategies for translating the basic
principle of multiple topological imaging into observational analysis are now
available, both for three-dimensional and two-dimensional catalogues.Comment: very compact, (2 page) write-up of invited review for Marcel
Grossmann IX Meeting in Roma, 1st page classifies and cites different
observational strategies, 2nd page reference list includes many key 1999/2000
reference
