Implied promise to indemnify surety permits recovery from principal debtor of sums rightfully paid under guarantee. In a contract of guarantee the principal debtor is implicitly bound to indemnify the surety for sums the surety rightfully pays under the guarantee; the surety may recover amounts properly paid but not sums paid wrongfully. Recoverable costs hinge on the reasonableness of the surety's defence or conduct, and recovery is confined to the principal debtor's actual liability under the guarantee.