Diagram perserved while adding annotations

digraph simple_hierarchy {
    a->b->c;
    b->d;
    a->e->f;
    e->g;
 
    // Drawing edges while preserving the tree layout with constraint=none
    // Very good for annotations.
    edge [color=red,arrowhead=none,constraint=none];
    b->f
    b->g
    d->f
    g->c
}