1. A method for verifying a specification, comprising the steps of: (a) retaining, by a processor of a computer system, a plurality of abstract documents, each said abstract document of said plurality indicating a value corresponding to a metadata of that document; (b) separating, by the processor, a group of abstract documents based on an input condition of an operation; (c) adding, by the processor, a new abstract document to said group by using, based on an output condition, at least one said operation within a group of said operations; (d) separating, by the processor, said abstract documents according to overlapping ranges of values designated by each said metadata of abstract documents indicated in said group; (e) unifying, by the processor, said abstract documents according to overlapping ranges of values designated by each said metadata of abstract documents indicated in said group; (f) repeating said steps (b) to (e) until a termination condition is satisfied; and (g) verifying, by the processor, whether an incomplete abstract document exists when said termination condition is satisfied; wherein said plurality of abstract documents associated with a plurality of said metadata are processed; wherein said group of said operations are applied collectively by an operation specification specifying document processing operations; wherein said input condition that is a condition of a range of values of said metadata capable of application of a respective operation is retained for each of said operation; and wherein an output condition that is a change of said metadata value after use of said respective operation is retained for each of said operation.