My Emacs version is 23.3.1 (i486-pc-linux-gnu, GTK+ Version 2.24.3), at least it says that. It is a standard package from Debian Linux (Wheezy), the GUI one.
nil does nothing.
What I want exactly: Insert a tab if and only if I press the TAB key. The "if" part works, but the "only if" part does not. I wonder why it's so hard?