 Another approach to the study of subsets of the continuum, or of a topological space in general, has appeared through the development of formal or abstract topology (Fourman 1982, Martin-Löf 1970, Sambin 1987). In this constructive topology the role of open sets and points is reversed; in classical topology an open set is defined as a certain set of points, in the constructive case open sets are the fundamental notion and points are defined in terms of them. Therefore this approach is sometimes referred to as point-free topology.