--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolbox/sub.m	Wed May 06 14:56:53 2015 +0200
@@ -0,0 +1,17 @@
+function S = sub(A, i, j)
+%SUB     Principal submatrix.
+%        SUB(A,i,j) is A(i:j,i:j).
+%        SUB(A,i)  is the leading principal submatrix of order i,
+%        A(1:i,1:i), if i>0, and the trailing principal submatrix
+%        of order ABS(i) if i<0.
+
+if nargin == 2
+   if i >= 0
+      S = A(1:i, 1:i);
+   else
+      n = min(size(A));
+      S = A(n+i+1:n, n+i+1:n);
+   end
+else
+   S = A(i:j, i:j);
+end