23. A system comprising: a tangible computer readable medium storing instructions that when executed by one or more processors cause the 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 phrase and one or more processors configured for executing the instructions stored on the computer readable storage medium.
25. The system of claim 23 , wherein associating the current top phrase and replacement top phrase with each other comprises: adding the current top phrase to a related phrase list of the replacement top phrase; and adding the replacement top phrase to a related phrase list of the current top phrase.