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.
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. 38. The computer readable storage medium of claim 37 , wherein associating the replacement top phrase with the root document of the document collection comprises adding the root document of the document collection to a posting list for the replacement top phrase.