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We note that, although, in [Z], Chen
uses a slightly different model for the **G**-module, his module is
readily seen to be isomorphic to ours.
Translated into our setting,
Chen proves the following:
Thus, in order to prove Theorem 1.1, it will suffice
to establish that there are
no further **G**-orbits in than those listed in Table 1, and that
is .
We shall however give a fully self-contained proof of Theorem 1.1.

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