workaround for PR#15325

git-svn-id: https://svn.r-project.org/R/trunk@63322 00db46b3-68df-0310-9c12-caf00c1e9a41
say a bit more about coercion

git-svn-id: https://svn.r-project.org/R/trunk@63324 00db46b3-68df-0310-9c12-caf00c1e9a41
incorporate PR#15274

git-svn-id: https://svn.r-project.org/R/trunk@63326 00db46b3-68df-0310-9c12-caf00c1e9a41