1. A method for abstracting/detailing structuring elements of system specification information, wherein specification information is represented by a hierarchical tree-structure having stories of structuring elements at various levels, the method comprising the steps of: selecting a first structuring element which is to be subjected to an abstracting or detailing process, the first structuring element being at a first level; selecting between an abstracting process and a detailing process; performing, if an abstracting process is selected, the steps of shifting the first structuring element to a second level lower than the first level, creating a second structuring element representative of the first structuring element, and placing the second structuring element at the first level into a position vacated by the first structuring element; and performing, if a detailing process is selected, the steps of deleting the first structuring element, shifting a third structuring element, which is representative of the first structuring element and located at a second level lower than the first level, to the first level into a position vacated by the first structuring element.