docs: public CharBuffer append (CharSequence csq, int start, int end) {
          if (csq == null) {
              csq = "null"; //$NON-NLS-1$
          }
          CharSequence cs = csq.subSequence(start, end);
          if (cs.length() > 0) {
              return put(cs.toString());
          }
          return this;
      }