 It is unsurprising, then, that certain conduct may constitute both a breach of contract and a delict (as when, in "Van Wyk v Lewis", a surgeon negligently left a cotton swab inside a patient's body), in which case there is concurrent liability, permitting the plaintiff to sue on either basis.Another source of obligations is quasi-contract, in which rights and duties arise from something other than "consensus".One example is unjustified enrichment, which occurs when wealth shifts from one person's patrimony to another's without legal justification.