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diff --git a/progs/demo/X11/animation/r_inbetween.hs b/progs/demo/X11/animation/r_inbetween.hs
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+{-******************************************************************
+  MODULE R_INBETWEEN
+
+    This module takes care of interpolation functions. Basically,
+  given two Pics, inbetween will give you a movie gradually 
+  converting from one Pic to the other Pic, using linear interpolation.
+    Tween will take two Movies, and append them, interpolating n
+  frames between the last Pic of the first Movie and the first Pic of
+  the last Movie.      
+
+******************************************************************-}
+
+module R_Inbetween (inbetween,tween) where
+
+import R_Ptypes
+import R_Utility
+import R_Picture
+import R_Behaviour
+
+  -- inbetween takes an int and two Pics, and interpolates n Pics
+  -- of interpolated Pics. 
+inbetween :: Int -> Pic -> Pic -> Movie
+inbetween n p1 p2 | (length p1 == length p2) = 
+                       ((zip1.(map (inbetweenp n))).zip1) [p1,p2]
+inbetween  n p1 p2       = inbetween n [(col,p1')] [(col,p2')]
+                          where p1' = concat [ vs | (c,vs) <- p1]
+                                p2' = concat [ vs | (c,vs) <- p2]
+                                col = head [ c | (c,vs) <- p1 ]
+                      
+  -- inbetweenp takes a list of 2 Polygons ([[Vec]]) and returns a 
+  -- sequence of interpolated Polygons. Should the Number of vertices 
+  -- of one Polygon be less than those in the other, it splits it so 
+  -- as to have two Polygons of the same length.
+inbetweenp :: Int -> Pic -> Pic
+inbetweenp n  [(c1,vs),(c2,ws)] = 
+   if ((length vs) < (length ws)) then  
+         inbetween1 (split (length ws) (c1,vs)) (c2,ws) 0 n
+   else if ((length vs) > (length ws)) then
+         inbetween1 (c1,vs) (split (length vs) (c2,ws)) 0 n
+   else inbetween1 (c1,vs) (c2,ws) 0 n
+                         
+
+  -- inbetween1 returns a sequence of interpolated Polygons.
+inbetween1 :: Poly -> Poly -> Int -> Int -> Pic
+inbetween1 p1 p2 m n | m>n || n<=0 = []
+inbetween1 p1 p2 m n               = inbetween2 p1 p2 m n 
+                                     :inbetween1 p1 p2 (m+1) n
+
+  -- inbetween2 returns ONE of the required sequence of 
+  -- interpolated Polygons.
+inbetween2 :: Poly -> Poly -> Int -> Int -> Poly
+inbetween2 (c1,vs) (c2,ws) p q = (c1, map (partway p q) (zip1 [vs,ws]))
+          
+  -- split splits up a Polygon so as to have the given #vertices.
+split :: Int -> Poly -> Poly
+split n (c,vs) = (c, split' n vs)
+
+split' :: Int -> [Vec] -> [Vec]
+split' n vs | n<= (length vs) = vs
+split' n vs = if (n>double) then
+                     split' n (split' double vs)
+              else 
+                     v1:(mid v1 v2):(split' (n-2) (v2:vss))
+              where double = 2*((length vs)) - 1
+                    (v1:v2:vss) = vs
+
+                           
+  -- tween will interpolate n Pics transforming the last Pic of
+  -- the first Movie into the first Pic of the second Movie, and
+  -- then run the second Movie    
+tween :: Int -> Movie -> Movie -> Movie         
+tween n m1 []   = m1
+tween n m1 m2  = m1 ++ (inbetween n (last m1) (head m2')) ++ (tail m2')       
+            where m2' = apply (mov (repeat v)) m2 
+                  v = vmin (orig_Pic (last m1)) (orig_Pic (head m2))
+
+  -- tweens will take a list of Movies and append them all, interpolating
+  -- n frames between every Movie.
+tweens :: Int -> [Movie] -> Movie
+tweens n = foldr (tween n) [] 
+
+