State any three classical quasi-contract situations.

The concept: obligations not from agreement but from law — the law's intervention to prevent injustice: the duty imposed on a person to pay or restore, in the form of contract but without the contract's consent-base. "Quasi" because contract-shaped (a duty to pay or account) though no true promise was made; "indirect" because the obligation arises by operation of law alongside the parties' direct bargain. The classical settings (the list): Payment by mistake or under coercion — money paid on a non-existent obligation: recoverable; Necessaries supplied to an incompetent — the minor's-or-lunatic's estate charged (Unit 4's exception); Money paid by a third party on the defendant's behalf at his request — the reimbursement; The finder of lost goods — the duties-and-claims of the finder (Unit 13's full treatment); The unauthorized manager (negotiorum gestio, the civil law's beneficial intervention) — the stranger managing another's affairs in necessity: the reimbursement of the necessary expenses; The duties of persons receiving money-or-goods by mistake or coercion — the restoration rules.