This scene demonstrates the problem.
----
camera {
	location -z
	look_at 0
}

#declare _f = function { x + clock }

plane {
	-z, 0
	pigment {
		function { _f(x,y,z) }
		color_map {
			[0 color rgb x]
			[1 color rgb z]
		}
	}
	finish {ambient 1}
}
----
Try specifying a clock value with +k, and it has no effect on the scene.
Replacing the function with
   #declare _f = function { x }
and adding
   translate x*clock
to the plane results in the plane shifting, demonstrating that the clock 
value is being correctly read in the scene.  It just isn't being used in 
the function correctly.

Beta 27 on Windows XP Pro, running on an Intel C2D### [at] 24gHz w/ 3Gb RAM.
Release (3.6) does not demonstrate this flaw.

...Chambers

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