15. An apparatus according to claim 14 wherein said differential decoding means comprises means responsive to at least one of i. a differential coding which coding is explicitly representative of an alphanumeric character of said first linguistic expression, and ii. a differential coding which is indirectly representative of a character sequence of said first linguistic expression, said character sequence being common to said first linguistic expression and a third linguistic expression represented in a third lexicon entry for generating signals representative of a character sequence of said first expression.
17. An apparatus according to claim 15 wherein said common expression verification means comprises means for generating a first hash code representative of said suspect expression.