Returning a template depending on arguments

Dear all,
this class:

template<int m, int n>
class SmallMatrix
{
    typedef double myData [m] [n];
    myData M;

public:
    SmallMatrix () {
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                M[i][j] = 0.;
            }
        }
    }
    virtual ~SmallMatrix () {}
    operator myData & () {
        return M;
    }
        operator const myData & () const {
        return M;
    }
};

compiles fine with g++-4.4.3 and does what I expect it to do, eg I can
declare a SmallMatrix<3,4> m; and then access its elements as m[i][j].

Then I tried to add an operator* like this:

    SmallMatrix<m, n1> operator* (const SmallMatrix<n, n1> & f) const {
        SmallMatrix<m, n1> result;
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n1; j++) {
                result[i][j] = 0.;
                for (int k = 0; k < n; k++) {
                    result[i][j] += M[i][k] * f[k][j];
                }
            }
        }
        return result;
    }

what I would like to do is:

SmallMatrix<2, 3> m23;
SmallMatrix<3, 4> m34;
SmallMatrix<2, 4> m24;
....
m24 = m23 * m34;

but the class SmallMatrix with the operator* method in it does not
compile; I get:
 'n1' was not declared in this scope
template argument 2 is invalid

clearly because n1 is not known before it appears in the return type
declaration; it becomes known only after the argument of operator* is
parsed.

I cannot see any way of getting what I would like to have. Is it
possible or it is one of the things one cannot do with c++?

TYIA