10. The system according to claim 9 , wherein the structured document processing apparatus further comprises: a broadening unit configured to broaden a range until a lexical item having not less than a frequency of occurrence is present within the range; and an assignment unit configured to assign a lexical identifier of a lexical item which has a highest frequency of occurrence within the broadened range as a relevant lexical identifier of the lexical item.
11. The system according to claim 10 , wherein the structured document processing apparatus further comprises: a storage unit configured to store respective relevant lexical identifiers of the lexical items; an assignment unit configured to assign structure model tree identifiers to nodes of the structure model tree; an assignment unit configured to assign a structure model tree identifier of a node where the relevant lexical identifier of a lexical item occurs most frequently as a relevant structure model tree identifier; and a storage unit configured to store respective relevant structure model tree identifiers of the lexical items.