def divide_and_conquer( S ):
    # (1). Divide the problem S into a set of subproblems {S1, S2, ... Sn}.
    [S1, S2, ... Sn] = divide(S)

    # (2). Solve the subproblem recursively,
    #   obtain the results of subproblems as [R1, R2... Rn].
    rets = [divide_and_conquer(Si) for Si in [S1, S2, ... Sn]]
    [R1, R2,... Rn] = rets

    # (3). combine the results from the subproblems.
    #   and return the combined result.
    return combine([R1, R2,... Rn])