template <class Scalar> void join_vector(
     VECTOR(Scalar)& xy, const VECTOR(Scalar)& x, const VECTOR(Scalar)& y
)
{    size_t n = size_t( x.size() );
     size_t m = size_t( y.size() );
     for(size_t i = 0; i < n; i++)
          xy[i] = x[i];
     for(size_t i = 0; i < m; i++)
          xy[n + i] = y[i];
}