1. A method for constructing a dictionary to be employed in compressing first textual data comprising: generating a plurality of keys of the dictionary, from second textual data, each key associated with an identifier, the keys comprising: a plurality of static word or phrase keys, each static word or phrase key listing one or more unchanging words in a particular order; and, a plurality of dynamic phrase keys, each dynamic phrase key listing a plurality of words and one or more placeholders in a particular order, each placeholder denoting a place where a word or phrase other than the words of the dynamic phrase key is to be inserted; and, storing the plurality of keys, including the identifiers of the keys.