  The biclique covering number of a bipartite graph G is the minimum number of
complete bipartite subgraphs (bicliques) whose union contains every edge of G.
  In this little note we compare three lower bounds on the biclique covering
number: A bound jk(G) proposed by Jukna & Kulikov (Discrete Math. 2009); the
well-known fooling set bound fool(G); the "tensor-power" fooling set bound
fool^\infty(G). We show jk \le fool le fool^\infty \le min_Q (rk Q)^2, where
the minimum is taken over all matrices with a certain zero/nonzero-pattern.
Only the first inequality is really novel, the third one generalizes a result
of Dietzfelbinger, Hromkovi\v{c}, Schnitger (1994).
  We also give examples for which fool \ge (rk)^{log_4 6} improving on
Dietzfelbinger et al.
