9. A computer-implemented method comprising: receiving, by at least one processor of a computer system, a user request to change a current top phrase for a limited document collection to a replacement top phrase; associating, by at least one processor of the computer system, the replacement top phrase with a root document of the document collection; associating, by at least one processor of a computer system, the current top phrase and the replacement top phrase with each other; adding, by at least one processor of the computer system, to phrase information for the replacement top phrase, phrase information for the current top phrase; and adding, by at least one processor of the computer system, to related phrase information of the replacement top phrase, related phrase information of the current top phrase.
9. A computer-implemented method comprising: receiving, by at least one processor of a computer system, a user request to change a current top phrase for a limited document collection to a replacement top phrase; associating, by at least one processor of the computer system, the replacement top phrase with a root document of the document collection; associating, by at least one processor of a computer system, the current top phrase and the replacement top phrase with each other; adding, by at least one processor of the computer system, to phrase information for the replacement top phrase, phrase information for the current top phrase; and adding, by at least one processor of the computer system, to related phrase information of the replacement top phrase, related phrase information of the current top phrase. 13. The method of claim 9 , 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.