Black to capture the two white stones at "a"

These are the intersections MadLab considers while searching:

The answer is Black "1" (see the figure below), which MadLab finds in about 30 seconds on a fast pc. After this, Black forces at "3" and "5", and jumps ahead to "7". No other black moves work. Note the timing of the jump: if it is performed before or after move 7, White will win.

Source: The problem comes from James Davies, Tesuji, Kiseido Publishing Company 1995, page 19.