extracting 'matrices out of matrices'



(forlack of a better descriptor)...

I'm working on a problem which yields a result of the form

[ [-203 -120]]
      45    -20

I can't quite draw it correctly here, but if double-click and copy from 
within wxMaxima, it yields the following

[matrix([-203,-120],[45,-20])]

I need to be able to invert the matrix, but can't figure out how to get 
the matrix 'out of' the first set of brackets (sorry, can't quite figure 
out the appropriate vernacular here...). The preceding construct looks 
like a matrix (2x2) within the single element of an outer (1x1) matrix.

Can anyone point out the obvious?

Cheers...