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.
41. The computer readable storage medium of claim 37 , wherein adding to related phrase information for the replacement top phrase, related phrase information of the current top phrase, comprises incrementing counts of related phrases of the replacement top phrase that are also related phrases of the current top phrase.