code: public static ErrorMessageFactory shouldContainOnlyOnce(String actual, String sequence, int occurences) {
      return new ShouldContainStringOnlyOnce(actual, sequence, occurences);
    }
docs: Creates a new <code>{@link ShouldContainStringOnlyOnce}</code>.
  
  @param actual the actual value in the failed assertion.
  @param sequence the String expected to be in {@code actual} only once.
  @param occurences the number of occurrences of sequence in actual.
  @return the created {@code ErrorMessageFactory}.