A3 Marking scheme for my part - 2b,c,d

2b.
    checking that all sets in F are vertex covers - 1p
                              and are minimal     - 1p
    checking certificate to be a vertex cover  - 1p
                               that is minimal - 1p
                               and not in F    - 1p

    arguing for IFF (correctness) - 2p
    running time - 2p
    length of certificate - 1p (could be part of running time)

    Common deductions:

	-5p if certificate was assumed to be the set of all minimal vertex
               covers, or the size of such a set
        -3p if checking of F missed (the first two checks above)


2c.
    finding smallest size of a vertex cover - 1p
    finding a smallest vertex cover -  5p
    some kind of invariant or argument for correctness - 2p
    running time - 2p

    Common deductions:

	-4 if nodes were removed and not put back

2d.
    case k= ... 1p
    case k> ... 4p - 2 for construction, 2 for IFF
    case k< ... 5p - 3 for construction, 2 for IFF

    Common deductions:

	-4 if case k< did not work because nodes were added to be
           connected somehow with nodes in G