@Test
public void testReplace() throws Exception {

    assertEquals("zarm", spelling.replaceCharAt("farm", 0, 'z'));
    assertEquals("fzrm", spelling.replaceCharAt("farm", 1, 'z'));
    assertEquals("fazm", spelling.replaceCharAt("farm", 2, 'z'));
    assertEquals("farz", spelling.replaceCharAt("farm", 3, 'z'));
}