provide formula interface as some expect it

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/curves-etc@297 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
get rid of new  "obsolete" warnings from gfortran

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/curves-etc@298 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
update

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/curves-etc@299 edb9625f-4e0d-4859-8d74-9fd3b1da38cb