16. The method of claim 1 in which the amended first customized document and the second customized document are associated with span information indicating the span of each portion of document content within the respective document.
18. The method of claim 16 in which the step of copying the portions of document content from the second customized document to the amended first customized document comprises the step of using the span information associated with the second customized document to determine the spans of the portions of document content in the second customized document to be copied.