code: public String fetch(int lineNumber) {
          E.illegalArgumentIf(lineNumber < 0, "line number must not be negative number: " + lineNumber);
          E.illegalArgumentIf(lineNumber >= lines(), "line number is out of range: " + lineNumber);
          List<String> list = fetch(lineNumber, 1);
          return list.isEmpty() ? null : list.get(0);
      }
docs: Returns the line specified by `lineNumber`.
  
  Note the `lineNumber` starts with `0`.
  
  @param lineNumber
  specify the line to be returned.
  @return the line as described above.