[sympy] tensor products and/or multidimensional matrix multiplication

```Greetings,

I am using sympy for the first time, and attempting to do matrix
multiplication like the following```
```
sum_{l,m} U[i,l] G[l,m] D[j,m]

I want to do regular matrix multiplication, but only on one of the two
indices for the objects U and D, in my case for a known G (I want to put
numerical entries for the matrix G).  I was hoping to have this print out a
symbolic expression.  I found sympy.MatrixSymbol to create a symbolic
representation for U and D, but it seems I need some functionality from
sympy.tensor class to handle this type of multiplication.

Does anyone have, or could anyone write down a simple example doing such a
multiplication?  Let me make a simple specific example.
Let

G = sympy.Matrix([[0,1], [-1,0]]) #
# [ 0  1 ]
# [ -1 0 ]

So, in the above example, leaving i,j arbitrary, this would result in

U[i,0] D[j,1] - U[i,1] D[j,0]

I also want to multiply this by a G[i,j], but knowing how to do the above I
think will be sufficient for the complete multiplication.

Thanks,
Andre

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