Syntax: LEVELS expr

   Set the levels for the contour command to be the elements of the
expression.  A vector's name is a simple expression, so the following
are all equivalent:

        SET levs = 1,4 SET levs = 2*levs LEVELS levs
	LEVELS { 2 4 6 8 }
        DEFINE four 4  LEVELS 2*< 1 2 3 $four >

