(;SZ[19]AW[qb][pc][qd][qe][mc]C[Black plays.

Source:
    "Good and Bad Style"
    Go World #10, p. 35]AB[qc][rc][pd][od]
(;B[oc]C[Bad style.];W[pb];B[ob]C[Black continues with another forcing move to create thickness, but ...];W[rb]TR[mc]C[... after White captures the corner here, Black's thickness has little effect on White's marked stone.])
(;B[pb]C[Correct.  Sacrificing another stone here significantly improves Black's result.];W[oc]
(;B[nc]C[Now Black can start the outside forcing moves.];W[ob];B[nb];W[pa];B[md]TR[mc]C[Black makes a nice shape that greatly affects the marked stone.

And later, Black has the following sequence in the corner ...];B[rb];W[ra];B[na];W[oa];B[sa]C[... and Black has a big multi-step ko that  White must worry about.])
(;B[ob]TR[qb]C[Capturing the marked stone is a mistake.];W[nc];B[rb]C[Black's relatively small corner doesn't match White's outside influence.])))