code: function(startRow, endRow, startCol, endCol) {
              	var x = [];
              	
              	if(endRow == 0)
              	    endRow = this.rows();
              	
              	if(endCol == 0)
              	    endCol = this.cols();
  
              	for(i = startRow; i <= endRow; i++) {
              	    var row = [];
  
              	    for(j = startCol; j <= endCol; j++) {
              		row.push(this.e(i, j));
              	    }
  
              	    x.push(row);
              	}
  
              	return $M(x);
                  }
docs: return a sub-block of the matrix