+Matrices often arise by omitting elements of another matrix. For
+instance, the submatrix @code{S} of a matrix @code{M} takes a
+rectangular block from @code{M}. The reduced matrix @code{R} is defined
+by removing one row and one column from a matrix @code{M}. (The
+determinant of a reduced matrix is called a @emph{Minor} of @code{M} and
+can be used for computing the inverse using Cramer's rule.)
+
+@cindex @code{sub_matrix()}
+@cindex @code{reduced_matrix()}
+@example
+ex sub_matrix(const matrix&m, unsigned r, unsigned nr, unsigned c, unsigned nc);
+ex reduced_matrix(const matrix& m, unsigned r, unsigned c);
+@end example
+
+The function @code{sub_matrix()} takes a row offset @code{r} and a
+column offset @code{c} and takes a block of @code{nr} rows and @code{nc}
+columns. The function @code{reduced_matrix()} has two integer arguments
+that specify which row and column to remove:
+
+@example
+@{
+ matrix m(3,3);
+ m = 11, 12, 13,
+ 21, 22, 23,
+ 31, 32, 33;
+ cout << reduced_matrix(m, 1, 1) << endl;
+ // -> [[11,13],[31,33]]
+ cout << sub_matrix(m, 1, 2, 1, 2) << endl;
+ // -> [[22,23],[32,33]]
+@}
+@end example
+
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