fix pdiff

contrib/math/TODO.txt

@@ -3,7 +3,7 @@ Nice to have:
 - Permutations -- composition and inverse (trivial; use sort and map-with)
 - Analysis:
   - error function, cosine integral, sine integral, fresnel functions
-  - exponential integral, logarithm integral, zeta function
+  - logarithm integral, zeta function
   - airy wave function
   - hypergeometric functions
   - elliptic functions

contrib/math/polynomials.factor

@@ -98,4 +98,4 @@ IN: math-contrib
 
 : pdiff ( p -- p' )
     #! Polynomial derivative.
-    [ length reverse-slice ] keep [ 1+ * ] 2map 1 swap head* ;
+    dup empty? [ [ length ] keep v* 1 swap tail ] unless ;