public static String convertToLowerUnderscore(String identifierName) {
    return splitToLowercaseTerms(identifierName).stream().collect(Collectors.joining("_"));
  }
@Test
  public void convertToLowerUnderscore_givesSingleUnderscore_fromSingleUnderscore() {
    String identifierName = "_";

    String lowerUnderscore = NamingConventions.convertToLowerUnderscore(identifierName);

    assertThat(lowerUnderscore).isEqualTo("_");
  }