A distance, per definition has the following 3 properties:
the distance between A and A is 0, or: D(A,A) = 0,
the distance between A and B is greater than or equal to 0, or: D(A,B) >= 0 and
the distance between A and C is smaller than or equal to the distance between A and B plus the distance between B and C, or D(A,C) <= D(A, B) + D(B, C).
