def index(self, x):
        '''
        Returns the first index i in the linked list, where the value x is found. Returns None if none of the Cells
        has a value of x.

        The first element in the linked list has the index 0.
        '''

        def indx(cell, n):
            if not cell.is_empty:
                return None
            if cell.value == x:
                return n
            return indx(cell.tail, n + 1)

        return indx(self, 0)