code: public void insert(StringBuilder buffer, Object value, int index) {
          if (buffer == null) {
              throw new NullPointerException("buffer");
          }
  
          buffer.insert(index, value);
      }
docs: A function to insert a value into an existing buffer.
  
  @param buffer  the buffer to insert into
  @param value  the value to insert
  @param index  the index of the position to insert at