14. A two-level document processing method preparing an upstream document on the basis of a downstream document, said method comprising the steps of: comparing the downstream document with an upstream addition document including information which is particular to the upstream document and is unnecessary for the downstream document; selecting, from the upstream addition document and based on results of the step of comparing, an addition candidate to be added to the downstream document until no more addition candidate is found; and preparing the upstream document by performing addition or non-addition of the addition candidate to the downstream document in accordance with results of the step of selecting.
14. A two-level document processing method preparing an upstream document on the basis of a downstream document, said method comprising the steps of: comparing the downstream document with an upstream addition document including information which is particular to the upstream document and is unnecessary for the downstream document; selecting, from the upstream addition document and based on results of the step of comparing, an addition candidate to be added to the downstream document until no more addition candidate is found; and preparing the upstream document by performing addition or non-addition of the addition candidate to the downstream document in accordance with results of the step of selecting. 17. The two-level document processing method as claimed in claim 14, wherein said step of preparing comprises the steps of: searching the upstream addition document for corresponding information corresponding to the addition candidate; and storing the addition candidate into an upstream special candidate file without adding the addition candidate to the downstream document, if the corresponding information corresponding to the addition candidate is absent in the upstream addition document.