43. The computer readable storage medium of claim 37 , further comprising instructions that when executed by one or more of the processors cause the system to decrement the counts of the related phrases of the replacement top phrase, responsive to the current and replacement top phrases having no related phrases in common.
37. A tangible computer readable storage medium comprising instructions that when executed by one or more processors cause a computer system to: receive a user request to change a current top phrase for a limited document collection to a replacement top phrase; associate the replacement top phrase with a root document of the document collection; associate the current top phrase and the replacement top phrase with each other; add to phrase information for the replacement top phrase, phrase information for the current top phrase; and add to related phrase information of the replacement top phrase, related phrase information of the current top.