7. The computer-assisted method of claim 6 , wherein the data set is obtained by the following steps: tokenizing a second group of text units in the external documents to produce a plurality of tokens comprising the jth token; for each text unit in the second group of text units that includes the jth token, adding a weighting coefficient to a parameter token_j_count, wherein the value of the weighting coefficient is dependent on the grammatical role of the jth token; and calculating an external term prominence value (ETP) for the jth token by dividing a cumulative value of the parameter token_j_count obtained from the second group of text units by the total number of text units in the second group of text units.
7. The computer-assisted method of claim 6 , wherein the data set is obtained by the following steps: tokenizing a second group of text units in the external documents to produce a plurality of tokens comprising the jth token; for each text unit in the second group of text units that includes the jth token, adding a weighting coefficient to a parameter token_j_count, wherein the value of the weighting coefficient is dependent on the grammatical role of the jth token; and calculating an external term prominence value (ETP) for the jth token by dividing a cumulative value of the parameter token_j_count obtained from the second group of text units by the total number of text units in the second group of text units. 8. The computer-assisted method of claim 7 , wherein the values of the weighting coefficients are constant for different tokens.