def add(self, key):
        """
        Add `key` as an item to this OrderedSet, then return its index.

        If `key` is already in the OrderedSet, return the index it already
        had.

        Example:
            >>> oset = OrderedSet()
            >>> oset.append(3)
            0
            >>> print(oset)
            OrderedSet([3])
        """
        if key not in self.map:
            self.map[key] = len(self.items)
            self.items.append(key)
        return self.map[key]

        Add `key` as an item to this OrderedSet, then return its index.

        If `key` is already in the OrderedSet, return the index it already
        had.

        Example:
            >>> oset = OrderedSet()
            >>> oset.append(3)
            0
            >>> print(oset)
            OrderedSet([3])
        
product/gradle-plugin/src/main/python/setuptools/_vendor/ordered_set.py