further cleanup of XML signature; replaced plain_content by incremental add_content; added stream output;
further cleanup of XML signature;
replaced plain_content by incremental add_content;
added stream output;

MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle