math.extras: adding the Kahan summation.

extra/math/extras/extras-docs.factor

@@ -86,3 +86,7 @@ HELP: round-to-decimal
     { $example "USING: math.extras prettyprint ;" "1.23456 2 round-to-decimal ." "1.23" }
     { $example "USING: math.extras prettyprint ;" "12345.6789 -3 round-to-decimal ." "12000.0" }
 } ;
+
+HELP: kahan-sum
+{ $values { "seq" sequence } { "n" float } }
+{ $description "Calculates the summation of the sequence using the Kahan summation algorithm." } ;

extra/math/extras/extras.factor

@@ -289,3 +289,8 @@ M: real round-away-from-zero
             [ swapd @ [ 1 + ] [ max 0 ] if ] keep swap
         ] reduce nip max
     ] if ; inline
+
+: kahan-sum ( seq -- n )
+    [ 0.0 0.0 ] dip [
+        rot - 2dup + [ -rot [ - ] bi@ ] keep
+    ] each nip ;