added clarifying comment

git-svn-id: http://haxe.googlecode.com/svn/trunk@6365 f16182fa-f095-11de-8f43-4547254af6c6
made invalid calls on abstracts more robust

git-svn-id: http://haxe.googlecode.com/svn/trunk@6366 f16182fa-f095-11de-8f43-4547254af6c6