Ocean Traders of North America is a company based in Mobile, Alabama, that specializes in seafood exports and commonly uses letters of credit (L/Cs) to ensure payment. It recently experienced a problem, however, Ocean Traders had an irrevocable L/C issued by a Russian bank to ensure that it would receive payment upon shipment of 16,000 tons of fish to a Russian firm. This bank backed out of its obligation, however, stating that it was not authorized to guarantee commercial transactions. Explain the function of irrevocable L/C for the case above.