<Tr> <Td_colspan="2"> Carlill v Carbolic Smoke Ball Co (1892) EWCA Civ 1 </Td> </Tr> <Tr> <Td_colspan="2"> Combe v Combe (1952) EWCA Civ 7 </Td> </Tr> <Tr> <Td_colspan="2"> Pao On v Lau Yiu Long (1979) UKPC 2 </Td> </Tr> <Tr> <Td_colspan="2"> Atlas Express Ltd v Kafco (1989) QB 833 </Td> </Tr>

What is a requirement of an express contract