10. The machine-readable storage device of claim 9 , wherein said operation of determining the frequency of use of the expression comprises: calculating a percentage of the occurrences of the expression in a set of predefined sample texts.
11. The machine-readable storage device of claim 10 , wherein said operation of associating each expression with the difficulty index further comprises: calculating a further percentage of the occurrences of the expression in the text, and updating the frequency of use of the expression according to the further percentage.