move nth-root to math.functions, use exchange-unsafe in math.statistics

basis/math/functions/functions.factor

@@ -106,6 +106,8 @@ PRIVATE>
         [ ^complex ]
     } cond ; inline
 
+: nth-root ( n x -- y ) swap recip ^ ; inline
+
 : gcd ( x y -- a d )
     [ 0 1 ] 2dip (gcd) dup 0 < [ neg ] when ; foldable
 
@@ -304,4 +306,3 @@ M: real atan >float atan ; inline
     [ [ / floor ] [ * ] bi ] unless-zero ;
 
 : lerp ( a b t -- a_t ) [ over - ] dip * + ; inline
-

basis/math/statistics/statistics.factor

@@ -1,8 +1,8 @@
 ! Copyright (C) 2008 Doug Coleman, Michael Judge.
 ! See http://factorcode.org/license.txt for BSD license.
-USING: arrays combinators kernel math math.analysis
-math.functions math.order sequences sorting locals
-sequences.private assocs fry ;
+USING: arrays combinators kernel math math.functions
+math.order sequences sorting locals sequences.private
+assocs fry ;
 IN: math.statistics
 
 : mean ( seq -- x )
@@ -33,7 +33,7 @@ IN: math.statistics
             [ i seq nth-unsafe x < ] [ i 1 + i! ] while
             [ x j seq nth-unsafe < ] [ j 1 - j! ] while
             i j <= [
-                i j seq exchange
+                i j seq exchange-unsafe
                 i 1 + i!
                 j 1 - j!
             ] when

extra/math/analysis/analysis.factor

@@ -56,9 +56,6 @@ PRIVATE>
         [ abs gammaln-lanczos6 ] keep dup 0 > [ drop ] [ gamma-neg ] if
     ] if ;
 
-: nth-root ( n x -- y )
-    swap recip ^ ;
-
 ! Forth Scientific Library Algorithm #1
 !
 ! Evaluates the Real Exponential Integral,