String distances that implement this interface are metrics, which means:
d(x, y) ≥ 0 (non-negativity, or separation axiom)
d(x, y) = 0 if and only if x = y (identity, or coincidence axiom)
d(x, y) = d(y, x) (symmetry)
d(x, z) ≤ d(x, y) + d(y, z) (triangle inequality).