A single valued function D(x, y) is a metric for a topological space provided
that for points x, y, z of the space:
1. D(x, y) ≽ 0, the equality holding if and only if x = y,
2. D(x, y) = D(y, x) (symmetry),
3. D(x, y) + D(y, z) ≽ D(x, z) (triangle inequality),
4. x belongs to the closure of the set M if and only if D(x, m) (m element of M) is not bounded from 0 (preserves limit points).