19. A computer system comprising: one or more memories configured for storing executable instructions; and one or more processors configured for executing the instructions, wherein the instructions include instructions to: determine, for a document that contains a first phrase, an expected number of related phrases that are related to the first phrase and are expected to be present in the document; determine for the document, and for the first phrase in the document, an actual number of related phrases present in the document; and identify the document as a spam document by comparing the actual number of related phrases present in the document with the expected number of related phrases, wherein determining the expected number of related phrases includes: traversing an index of a plurality of documents; for each of the indexed documents, determining a set of phrases in the document, and for each phrase in the set, determining a number of related phrases also in the document; and determining the expected number of related phrases based on the determined number of related phrases across the traversed documents.
19. A computer system comprising: one or more memories configured for storing executable instructions; and one or more processors configured for executing the instructions, wherein the instructions include instructions to: determine, for a document that contains a first phrase, an expected number of related phrases that are related to the first phrase and are expected to be present in the document; determine for the document, and for the first phrase in the document, an actual number of related phrases present in the document; and identify the document as a spam document by comparing the actual number of related phrases present in the document with the expected number of related phrases, wherein determining the expected number of related phrases includes: traversing an index of a plurality of documents; for each of the indexed documents, determining a set of phrases in the document, and for each phrase in the set, determining a number of related phrases also in the document; and determining the expected number of related phrases based on the determined number of related phrases across the traversed documents. 28. The computer system of claim 19 , wherein the instructions further include instructions to: add the identified document to a spam list that includes a list of spam documents.