some bug fixes.
add suppport for more than one conditional variable.
polish argument names.



git-svn-id: svn://svn.r-forge.r-project.org/svnroot/vcd@764 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
remove conditional variables where all obs are identical (resulting e.g. from a subset)



git-svn-id: svn://svn.r-forge.r-project.org/svnroot/vcd@765 edb9625f-4e0d-4859-8d74-9fd3b1da38cb