code: func (c *configuration) stackValue(i int) head {
  	size := c.stackSize()
  	if i >= size || i < 0 {
  		return DOES_NOT_EXIST
  	}
  	return c.stack[size-1-i]
  }
docs: // gets the sentence index of the ith word on the stack. If there isn't anything on the stack, it returns DOES_NOT_EXIST