A metric function defines a distance between elements of a set. Besides
non-negativity, isolation, and symmetry, it also has to satisfy triangular
inequality.
- non-negativity: d(x, y) ≥ 0
- isolation: d(x, y) = 0 if and only if x = y
- symmetry: d(x, y) = d(x, y)
- triangular inequality: d(x, y) + d(y, z) ≥ d(x, z).
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