1. A program embodied in computer readable medium for controlling a computer so that the computer performs data processing for addressing at least one predetermined element in a structured document, the program causing the computer to perform: first processing of, when the structured document having the element addressed by predetermined addressing information is modified, inputting the structured document to analyze the modification and storing an analysis result in a memory; and second processing of reading the analysis result from the memory and updating the addressing information according to the analyzed modification so that the addressing information addresses at least one corresponding element in the modified structured document; and wherein the first processing provided by the program comprises the processing of: converting an unmodified version and a modified version of the structured document into tree-structured data items; and computing a difference between the tree-structured data items to track a component of the tree-structured data items that is moved in operations required for transformation between the tree-structured data items transformed from one to the other according to modification of the structured document; and wherein in the second processing provided by the program, the program causes the computer to update the addressing information based on the difference between the tree-structured data items.