6. The computer system of claim 5 , wherein the confidence assignor assigns the confidence scores for the respective messages by: for respective word sequences, identifying the count of the occurrences of the word sequence among the messages of the respective contexts over time; and assigning the confidence scores to the respective word sequences according to the count over time of the occurrences of the respective word sequences among the messages of the respective contexts.
6. The computer system of claim 5 , wherein the confidence assignor assigns the confidence scores for the respective messages by: for respective word sequences, identifying the count of the occurrences of the word sequence among the messages of the respective contexts over time; and assigning the confidence scores to the respective word sequences according to the count over time of the occurrences of the respective word sequences among the messages of the respective contexts. 9. The computer system of claim 6 , wherein the lexicon generator generates the lexicon for the context by: ordering the respective word sequences within the context according to the respective confidence scores; and choosing a top percentage of the word sequences having highest confidence scores among the messages of the context.