18. The system of claim 17 in which calculating a tf-idf value of each extracted constituent term further comprises: calculating an inverse document frequency (idf) value of the constituent term in relation to the corpus of documents; calculating a term frequency (tf) value of the constituent term; and determining the tf-idf value of the extracted constituent term based on the idf value of the constituent term and the tf value of the constituent term.
18. The system of claim 17 in which calculating a tf-idf value of each extracted constituent term further comprises: calculating an inverse document frequency (idf) value of the constituent term in relation to the corpus of documents; calculating a term frequency (tf) value of the constituent term; and determining the tf-idf value of the extracted constituent term based on the idf value of the constituent term and the tf value of the constituent term. 20. The system of claim 18 in which calculating the tf value of the constituent term further comprises dividing a number of times the constituent term appears in the text by a length of the text.