15.43  INTERPOLATE 

INTERPOLATE [LEVEL=n] [{SPLINE|GENERAL}] [CLOSED[=ON|OFF]]
     [POINTS|COLUMNS=[FROM] n1 [TO] [n2]]
     [SETS=[FROM] n1 [TO] [n2]]
     [SELECT="name"]
     [LIMITED [VLOG[=ON|OFF]] [[FROM]|TO [[X=]nx,[[Y=]ny[,[Z=]nz]]]
          [RECURSOR] [CURSOR] ]
     [APPEND[=ON|OFF]] [NAME="name"] [LOG[=ON|OFF]]
Interpolates a data set and puts the result into a new data set.  LEVEL is
the number of intervals per pair of  points.   If  2  a  single  point  is
interpolated in between each existing point.  If it is 5 then 4 points are
interpolated.  X, Y, and Z if available are all interpolated.

The  data  must  be  strictly  monotonic  in  either  Y  or X for a SPLINE
interplation.

If the first and last points of a curve are the same, the curve is assumed
to be closed, and the interpolation is done accordingly.  If the  data  is
POLAR or SPHERICAL then the angles must vary smoothly for proper closure.