When extending the data, keep missing values by using data.frame(, na.action=NULL)

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/r2mlwin@228 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
When extending the data, keep missing values by using data.frame(, na.action=NULL)

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/r2mlwin@229 edb9625f-4e0d-4859-8d74-9fd3b1da38cb