exrecises added

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sintro@251 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
exrecises added

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/sintro@252 edb9625f-4e0d-4859-8d74-9fd3b1da38cb