The right-hand side of equations17 and 18 contain sign errors in the term in the second row of the first column. The leading minus signs should be omitted. This error was not made in the calculations and did not affect the results. The corrected equations are:  
hp=[hxxpi^i^hxypi^j^hxzpi^k^hyxpj^i^hyypj^j^hyzpj^k^hz...
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