def insert(self, value):
        """Insert an occurrence of `value` into the btree."""
        i = 0
        n = len(self._tree)
        while i < n:
            cur = self._tree[i]
            self._counts[i] += 1
            if value < cur:
                i = 2 * i + 1
            elif value > cur:
                i = 2 * i + 2
            else:
                return
        raise ValueError("Value %s not contained in tree." "Also, the counts are now messed up." % value)
Insert an occurrence of `value` into the btree.