“…For k = 2, the condition τ n (X) = ∅ can be expressed in terms of sets of points in X that are geometrically explicit. In the special case of the algebras U and V considered above, these points were introduced in the literature under different names such as "permissible lines" [4] "bolts of lightning" [1,6,7,13,14,21,22], "trips" [20], "paths" [5,8,10,18,19], "links" [3,15], etc. The term bolt of lightning is the most common one and is due to Arnold [1].…”