J. Neggers and H. S. Kim introduced in [8]
the notion of d-algebras, i.e. algebras satisfying (1) xx = 0, (5) 0x = 0,
(6) xy = 0 and yx = 0 imply x = y, which is another useful generalization
of BCK-algebras, and then they investigated several relations
between d-algebras and BCK-algebras as well as some other interesting
relations between d-algebras and oriented digraphs.