<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def deselect_by_index(self, index):
 """Deselect the option at the given index. This is done by examing the "index" attribute of an element, and not merely by counting. :Args: - index - The option at this index will be deselected throws NoSuchElementException If there is no option with specified index in SELECT """

        if not self.is_multiple:
            raise NotImplementedError("You may only deselect options of a multi-select")
        for opt in self.options:
            if opt.get_attribute("index") == str(index):
                self._unsetSelected(opt)
                return
        raise NoSuchElementException("Could not locate element with index %d" % index)