code: public static Optional<String> substring(String str, char delimiterOfInclusiveBeginIndex) {
          if (N.isNullOrEmpty(str)) {
              return Optional.<String> empty();
          }
  
          return substring(str, str.indexOf(delimiterOfInclusiveBeginIndex));
      }
docs: Returns an empty <code>Optional</code> if {@code N.isNullOrEmpty(str) || str.indexOf(delimiterOfInclusiveBeginIndex) < 0},
  otherwise an {@code Optional} with String value: {@code str.substring(str.indexOf(delimiterOfInclusiveBeginIndex))} is returned.
  
  @param str
  @param delimiterOfInclusiveBeginIndex {@code inclusiveBeginIndex <- str.indexOf(delimiterOfInclusiveBeginIndex)}
  @return
  @see #substring(String, int)