US Patent 7278064 Information delivery system

An apparatus for processing digital data receives a first set of digital data that has a plurality of subsets of digital data. The apparatus contains memory to store the digital data. The apparatus receives a second set of digital data that has a first set of displayable data, a linkage reference and a second set of displayable data. The second set of digital data is used to indicate a presence of the linkage reference. The linkage reference is associated with one of the subsets of digital data. The apparatus determines whether there is error in the subsets of digital data. The apparatus causes a display device to display the second set of displayable data in a first way to indicate that the first linkage reference is active if there is no error and a second way to indicate that the first linkage reference is not active when there is error.