<Tr> <Td> </Td> <Td> Harold Wilson </Td> <Td> </Td> <Td> Edward Heath </Td> <Td> 1974--1975 </Td> </Tr> <Tr> <Td> Margaret Thatcher </Td> <Td> 1975--1976 </Td> </Tr> <Tr> <Td> James Callaghan </Td> <Td> 1976--1979 </Td> </Tr> <Tr> <Td> </Td> <Td> Margaret Thatcher </Td> <Td> </Td> <Td> James Callaghan </Td> <Td> 1979--1980 </Td> </Tr>

What does the prime minister do in a debate