public Collection removeLeftmostVertices(){
        Collection<Vertex2D> removed = new ArrayList<Vertex2D>();
        Vertex2D minX = vertices.get(0);
        for(int i = 0; i < vertices.size(); i++){
            if(vertices.get(i).getX() < minX.getX()){
                minX = vertices.get(i);
            }
        }
        removed.add(minX);
        for(int i = 0; i < vertices.size(); i++){
            if(vertices.get(i).getX() == minX.getX() && !vertices.get(i).equals(minX)){
                removed.add(vertices.get(i));
            }
        }

        return removed;