1. A method in a computer system for identifying passages of a first body of text relating to a passage of a second body of text, the method comprising the steps of: for each of a multiplicity of passages of the first body of text each having a location in the first body of text: constructing a first logical form characterizing a semantic relationship between selected words in the passage, expanding the constructed first logical form to include alternative words for at least some of the selected words in the passage, and storing in an index a mapping from the expanded first logical form to the location of the passage in the first body of text; constructing a second logical form characterizing a semantic relationship between selected words in the passage of the second body of text; expanding the constructed second logical form characterizing a semantic relationship between selected words in the passage of the second body of text to include alternative words for at least some of the selected words in the passage; and comparing the expanded second logical form characterizing a semantic relationship between selected words in the passage of the second body of text to the expanded first logical forms from which the index maps to identify a passage of the first body of text whose expanded logical form intersects with the expanded logical form characterizing a semantic relationship between selected words in the passage of the second body of text, in that, for pair of corresponding selected words between the intersecting expanded logical forms, the selected word or one of its alternative words in the expanded first logical form matches the selected word or one of its alternative words in the expanded second logical form, such that a passage of the first body of text relating to the passage of the second body of text is identified.