public void removeEdge(V from, V to) {
        if (!containsVertex(from)) {
            throw new IllegalArgumentException("Nonexistent vertex " + from);
        }

        if (!containsVertex(to)) {
            throw new IllegalArgumentException("Nonexistent vertex " + to);
        }

        neighbors.get(from).remove(to);
    }
@Test
    void removeEdge() {
        graph.removeEdge('B', 'F');
        List<Character> result = graph.getNeighbors('B');
        List<Character> expected = Arrays.asList('C');
        assertEquals(expected, result);
    }