Submatrices support all operations supported by the regular matrix classes. Because the matrix classes uniformly use only the entry() and operator [] member functions to access the data, the submatrix classes need only to override these functions, and all matrix operations become available on submatrices.
xxx& entry(int i)
Return the i-th entry of the submatrix when its data storage is considered to be a linear array.
xxx* operator [] (int row)
Return a pointer to the start of the row of the submatrices data storage.