20. The computer program product of claim 19 , wherein said plasticity module determines when one or more of said at least one keyword that is not tagged as a system keyword is promoted to system keyword status based on momentum, wherein momentum is the number of occurrences of the untagged keyword per unit time.
19. The computer program product of claim 18 , wherein said natural language processing module extracts said at least one keyword from said new documents and tags each of said at least one keyword that is a system keyword.