1. A method of mitigating the effect of a security threats potentially contained in a digitally encoded document, the method comprising: determining whether the digitally encoded document contains any embedded documents; extracting embedded content into a separate document, such that a single input digital encoded document is translated into parts which contain references to one another; content-checking, by means of at least one hardware-implement content-checker, at least one of the embedded documents separately from those parts of the digitally encoded document within which it was embedded; recombining at least some of the parts of the extracted embedded content into the digitally encoded document using the references contained therein; and releasing a version of the digitally encoded document responsive to the content-checking
2. A method according to claim 1 in which embedded documents are nested.