15.15.3  CYCLE 

CYCLE  causes  the  line texture, color, and width attributes to vary from
one set of secondary contour lines to the next.  The order of  cycling  is
set  by  the  SET  CYCLE  command.   The  attributes  specified by the SET
SECONDARY command override those  specified  by  the  SET  CYCLE  command.
Primary  contour  lines also vary according to cycles.  If you specify any
attributes they are used for the primary contours, and  override  the  the
cycles.