Its job is to enable and disable the mode by setting the variable.
The command should accept one optional argument. If the argument is
-@code{nil}, it should toggle the mode (turn it on if it is off, and off
-if it is on). Otherwise, it should turn the mode on if the argument is
-a positive integer, a symbol other than @code{nil} or @code{-}, or a
-list whose @sc{car} is such an integer or symbol; it should turn the
-mode off otherwise.
+@code{nil}, it should toggle the mode (turn it on if it is off, and
+off if it is on). It should turn the mode on if the argument is a
+positive integer, the symbol @code{t}, or a list whose @sc{car} is one
+of those. It should turn the mode off if the argument is a negative
+integer or zero, the symbol @code{-}, or a list whose @sc{car} is one
+of those. The meaning of other arguments is not specified.
Here is an example taken from the definition of @code{transient-mark-mode}.
It shows the use of @code{transient-mark-mode} as a variable that enables or