public void flip() {
Task tmp = v;
this.w.addEdge(this);
this.v.removeEdge(this);
this.v = w;
this.w = tmp;
w.addPredecessor();
v.removePredecessor();
}
Edge(Task v, Task w) {
this.v = v;
this.w = w;
w.addPredecessor();
}