previous addition broke example flow -> fixed now

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/zoo@996 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
better included stopifnot(require(...)) and ...::...

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/zoo@997 edb9625f-4e0d-4859-8d74-9fd3b1da38cb