14. An apparatus for facilitating computer-assisted comprehension of texts, the apparatus comprising: a microprocessor; and machine-readable storage medium coupled with the microprocessor, the machine-readable storage medium encoded with instructions executable by the microprocessor, the instructions configured to; determine a plurality of expressions in a document presented in a document viewing application, wherein the document includes at least one occurrence of each of the plurality of expressions; calculate a difficulty index for each of the plurality of expressions based, at least in part, on frequency of occurrence of the expression in the document, wherein a lower frequency of occurrence estimates a greater comprehension difficulty; determine a set of the plurality of expressions that have difficulty indices that exceed a threshold value; associate each of the set of the plurality of expressions with a corresponding explanation; and supply for display an indication of the corresponding explanation of each of the set of the plurality of expressions in a part of the document being displayed; determine a frequency of occurrence of a second plurality of expressions in a plurality of sample documents, wherein each of the second plurality of expressions occurs at least once in the plurality of sample documents; calculate an initial difficulty index, for each of the second plurality of expressions, that estimates comprehension difficulty for each of the second plurality of expressions, wherein the initial difficult index is calculated based, at least in part, on the frequency of occurrence of the second plurality of expressions in the plurality of sample documents; and wherein the instructions to calculate the difficulty index for each of the plurality of expressions comprises instructions to update the initial difficulty index, for each of those of the plurality of expressions also in the second plurality of expressions, based on the frequency of occurrence of the expression in the document.
14. An apparatus for facilitating computer-assisted comprehension of texts, the apparatus comprising: a microprocessor; and machine-readable storage medium coupled with the microprocessor, the machine-readable storage medium encoded with instructions executable by the microprocessor, the instructions configured to; determine a plurality of expressions in a document presented in a document viewing application, wherein the document includes at least one occurrence of each of the plurality of expressions; calculate a difficulty index for each of the plurality of expressions based, at least in part, on frequency of occurrence of the expression in the document, wherein a lower frequency of occurrence estimates a greater comprehension difficulty; determine a set of the plurality of expressions that have difficulty indices that exceed a threshold value; associate each of the set of the plurality of expressions with a corresponding explanation; and supply for display an indication of the corresponding explanation of each of the set of the plurality of expressions in a part of the document being displayed; determine a frequency of occurrence of a second plurality of expressions in a plurality of sample documents, wherein each of the second plurality of expressions occurs at least once in the plurality of sample documents; calculate an initial difficulty index, for each of the second plurality of expressions, that estimates comprehension difficulty for each of the second plurality of expressions, wherein the initial difficult index is calculated based, at least in part, on the frequency of occurrence of the second plurality of expressions in the plurality of sample documents; and wherein the instructions to calculate the difficulty index for each of the plurality of expressions comprises instructions to update the initial difficulty index, for each of those of the plurality of expressions also in the second plurality of expressions, based on the frequency of occurrence of the expression in the document. 15. The apparatus of claim 14 , wherein the corresponding explanation comprises at least one of a translation, a synonym, and a definition.