/*
             /-> B -\
begin -> A -<       >-> D -> E ->end
             \-> C -/

depenendencies  |  dependents
----------------|-------------
[E]             :  [end]
[D]             :  [E]
[B, C]          :  [D]
[A]             :  [B, C]
[begin]         :  [A]
*/

auto expand(node n)
{
    if(n == begin)
    {
        return n;
    }
    else if(is_linear_to(n))
    {
        return expand(only_dependency(n)).then(n);
    }
    else if(is_join)
    {
        return when_all(expand_fork(dependencies(n)...)).then(n);
    }
}

auto expand_fork(nodes... ns)
{
    return expand(fork_origin_of(ns...)).then_in_parallel(ns...);
}

// begin...
expand(end)

// is_linear_to(end)? true
expand(E).then(end)

// is_linear_to(E)? true
expand(D).then(E).then(end)

// is_join_to(D)? true
when_all(expand_fork(B, C)).then(D).then(E)).then(end)

// expand_fork(B, C)
when_all(expand(A).then_in_parallel(B, C)).then(D).then(E)).then(end)

// is_linear_to(A)? true
when_all(expand(begin).then(A).then_in_parallel(B, C)).then(D).then(E)).then(end)

// is_begin? true
when_all(begin.then(A).then_in_parallel(B, C)).then(D).then(E)).then(end)