@Override
    public Writer append(CharSequence csq) {
        if (csq == null) {
            write("null");
        } else {
            write(csq.toString());
        }
        return this;
    }
@Test
    void testAppend() {
        UnsafeStringWriter writer = new UnsafeStringWriter();
        writer.append('a');
        writer.append("abc", 1, 2);
        writer.append('c');
        writer.flush();
        writer.close();

        assertThat(writer.toString(), is("abc"));
    }