* dict_next() successor function, verifying that the key of each node is
* strictly lower than that of its successor, if duplicates are not allowed,
* or lower or equal if duplicates are allowed. This function is used for
* dict_next() successor function, verifying that the key of each node is
* strictly lower than that of its successor, if duplicates are not allowed,
* or lower or equal if duplicates are allowed. This function is used for