A compound verb meaning to add provisions to an existing arrangement, particularly a covenant or legal document. Galatians 3:15 uses it in Paul's argument: even a human covenant, once ratified, cannot be annulled or 'added to' (ἐπιδιατάσσεται). The verb suggests unauthorized modification or amendment. Paul uses this legal analogy to argue that the Mosaic law, coming 430 years after Abraham, cannot alter God's covenant promise.