for each arbitrary grouping (i.e. category), look at each child weakness, make sure that child has a parent that is a weakness, then remove the category from the view (typically this involved shifting the category relationships to the Development view).
when a weakness cannot be abstracted any more, it is treated as a Pillar - a root of the hierarchy.
At the release of CWE 1.0, only one category remains, which could not be resolved by release time because of the need for community involvement.
It should be noted that we did not merge pillars by following the "seven plus one" principle as used in 7PK. For example, both CWE-682 (Incorrect Calculation) and CWE-330 (Use of Insufficiently Random Values) are related to numbers, but they were not merged into one pillar, because it would force the creation of a category.