8. A method, comprising: obtaining, by a processor, a plurality of n-grams from a plurality of messages; determining, by the processor, a temporal histogram for each n-gram; determining, by the processor, synonyms among the n-grams based on a combination of a correlation of the histograms and a distance measure between n-grams; and selecting, by the processor, an n-gram from the synonyms, wherein the selection is based on a difficulty metric computed for each of a plurality of the n-grams.
8. A method, comprising: obtaining, by a processor, a plurality of n-grams from a plurality of messages; determining, by the processor, a temporal histogram for each n-gram; determining, by the processor, synonyms among the n-grams based on a combination of a correlation of the histograms and a distance measure between n-grams; and selecting, by the processor, an n-gram from the synonyms, wherein the selection is based on a difficulty metric computed for each of a plurality of the n-grams. 9. The method of claim 8 further comprising computing the combination of the correlation of the histograms and the distance measure by computing a weighted sum of the correlation of the histograms and the distance measure.