SORT:
    P = list of all plugins

    for each p in P do
        if color[a] == WHITE
            DFS-VISIT(p)

DFS-VISIT(Plugin p)
    A = p's after list.
    
    color[a] == BLACK

    for each a in A do:
        if color[a] == WHITE
            DFS_VISIT(a)
    add a to the list of the sorted plugins