Block literals as parameters to an enclosing block as a substitute for dispatch groups, allowing synchronization of an aggregate of tasks. There are two task aggregates: one supplies the value for d1; the other, c2; They are notated by 1 and 2, respectively; the letters (a through d, a through c) mark the progression of the tasks in each aggregate.

nested_block_literals.m

^(int d1, int(^c2)(void))
    {
        return c2;
        } (^(int c1)
           {
                return c1;
            } (^(int b1)
               {
                    return b1;
                } (a1)),
           ^(int b2)
           {
                return ^{
                            return b2;
                        };
           } (a2));