[1] Z. HEDRLÍN:
On universal partly ordered sets and classese. Journ. of Alg. 11, 4 (1969), 503-509.
MR 0238919
[2] P. HELL J. NEŠETŘIL:
Graphs and k-societies. Canad. Math. Bulletin 13, 3 (1970), 375-381.
MR 0276124
[3] L. KUČERA:
On universal concrete categories. to appear in Algebra Universalis 1 (1971).
MR 0404385