diagonals of tensors

[Thomas Lumley]

 Is there a simple and efficient way of extract `diagonals' from arrays
(tensors)

That is, suppose  A is a 20x10x2 array, B is 10x5 and C is 20x5 I would
like the 2x5 matrix whose i,j entry is

      sum_k sum_l  A(k,l,i)B(l,j)C(k,j)


Using tensor() (or outer and matrix multiplication) it's easy to construct
the 2x5x5 matrix with i,j,J entry

       sum_k sum_l  A(k,l,i)B(l,j)C(k,J)

[eg tensor(tensor(A,B,2,1),C,1,1) ]
and I'm looking for the j==J `diagonal', ideally without constructing the
whole thing.


	-thomas

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