matrix inside a matrix?

Is it possible to define a matrix that is part of a larger matrix, without
having to create temporaries and copy part of the larger?

/----------\
|    |     |
| SM |     |
|----| LM  |
|          |
\----------/

I need to extract a sub-section of a matrix, multiply it, and then copy it 
back. 
I am currently using a temporary, but I have a feeling there must be a better 
way.

-- 
Stephen

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