samer@4: % submat - extract square submatrix
samer@4: %
samer@4: % submat :: [[N,M]], [[L]->[min(N,M)]] -> [[L,L]].
samer@4: %
samer@4: % submat(X,I)=X(I,I)
samer@4: function B=submat(A,I), B=A(I,I); end