14. A non-transitory computer-readable recording medium having recorded thereon a software program used to carry out information processing on document collections having tags permitting semantic class identification appended to each document, the software program comprising instructions directing a computer to carry out a method comprising: creating a plurality of semantic class units containing two or more semantic classes based on a taxonomy that identifies relationships between semantic classes among a plurality of semantic classes, such that there is no semantic class in a semantic class unit which is an ancestor or a descendant of any other semantic class in the same semantic class unit; identifying, for each of the semantic class units, documents that match that semantic class unit in the document collections, and calculating a first frequency and a second frequency, the first frequency representing occurrence of the semantic class units in a first subset of documents of the document collections, and the second frequency representing occurrence of the semantic class units in a second subset of documents of the document collections; and searching in the document collections based on externally entered search terms, and separating the document collections into the first subset by determining that documents of the first subset comprise the search terms and into the second subset by determining that documents of the second subset do not comprise the search terms; and identifying the semantic class units and output the semantic class units to an enumeration tree in response to determining that the first frequency is higher than a first threshold value and that the second frequency is lower than a second threshold value, wherein the taxonomy identifies relationships between semantic classes among the plurality of semantic classes in a hierarchical manner, and wherein the method further comprises carrying out a child class conversion process and a class addition process in the enumeration tree whose root is a null set, the search unit creates an enumeration tree by designating one, two or more of the semantic classes as nodes and establishing links between the nodes, and extracts a combination of semantic classes in which the semantic classes are not an ancestor or a descendant of each other to create the semantic class units.
14. A non-transitory computer-readable recording medium having recorded thereon a software program used to carry out information processing on document collections having tags permitting semantic class identification appended to each document, the software program comprising instructions directing a computer to carry out a method comprising: creating a plurality of semantic class units containing two or more semantic classes based on a taxonomy that identifies relationships between semantic classes among a plurality of semantic classes, such that there is no semantic class in a semantic class unit which is an ancestor or a descendant of any other semantic class in the same semantic class unit; identifying, for each of the semantic class units, documents that match that semantic class unit in the document collections, and calculating a first frequency and a second frequency, the first frequency representing occurrence of the semantic class units in a first subset of documents of the document collections, and the second frequency representing occurrence of the semantic class units in a second subset of documents of the document collections; and searching in the document collections based on externally entered search terms, and separating the document collections into the first subset by determining that documents of the first subset comprise the search terms and into the second subset by determining that documents of the second subset do not comprise the search terms; and identifying the semantic class units and output the semantic class units to an enumeration tree in response to determining that the first frequency is higher than a first threshold value and that the second frequency is lower than a second threshold value, wherein the taxonomy identifies relationships between semantic classes among the plurality of semantic classes in a hierarchical manner, and wherein the method further comprises carrying out a child class conversion process and a class addition process in the enumeration tree whose root is a null set, the search unit creates an enumeration tree by designating one, two or more of the semantic classes as nodes and establishing links between the nodes, and extracts a combination of semantic classes in which the semantic classes are not an ancestor or a descendant of each other to create the semantic class units. 15. The non-transitory computer-readable recording medium according to claim 14 , wherein: the tags contain identifiers indicating the semantic classes corresponding thereto; and the method further comprises identifying, for each of the semantic class units, a set of identifiers corresponding to said semantic class unit, and identifying documents matching the semantic class units by comparing the set of the identified identifiers and the tags appended to each document in the document collections.