5. A computer storage medium encoded with instructions that when executed by one or more computers cause the one or more computers to perform operations comprising: storing, in an index associated with a document, a particular term that occurs in the document, wherein the particular term comprises quantity m words, wherein the quantity m is greater than 1; identifying a substitute term of the particular term, wherein the substitute term comprises quantity n separate words, and wherein the quantity n is greater than 1; and in response to identifying the substitute term of the particular term, storing, in the index associated with the document, (i) each of the quantity n words of the substitute term of the particular term as a separate index entry, and (ii) for each respective index entry, data indicating the quantity m and data indicating a value between 1 and n inclusive, wherein the value reflects an order of the respective word within the quantity n words of the substitute term.
5. A computer storage medium encoded with instructions that when executed by one or more computers cause the one or more computers to perform operations comprising: storing, in an index associated with a document, a particular term that occurs in the document, wherein the particular term comprises quantity m words, wherein the quantity m is greater than 1; identifying a substitute term of the particular term, wherein the substitute term comprises quantity n separate words, and wherein the quantity n is greater than 1; and in response to identifying the substitute term of the particular term, storing, in the index associated with the document, (i) each of the quantity n words of the substitute term of the particular term as a separate index entry, and (ii) for each respective index entry, data indicating the quantity m and data indicating a value between 1 and n inclusive, wherein the value reflects an order of the respective word within the quantity n words of the substitute term. 8. The storage medium of claim 5 , further comprising: storing, in the index associate with the document, information indicating that the substitute term of the particular term (i) is a substitute term of the particular term, or (ii) is not a term that occurs in the document.