Quick fix from HORN-9
diff --git a/src/main/java/org/apache/horn/funcs/Sigmoid.java b/src/main/java/org/apache/horn/funcs/Sigmoid.java
index 4472b8a..cc393e3 100644
--- a/src/main/java/org/apache/horn/funcs/Sigmoid.java
+++ b/src/main/java/org/apache/horn/funcs/Sigmoid.java
@@ -35,7 +35,7 @@
 
   @Override
   public double applyDerivative(double value) {
-    return value * (1 - value);
+    return apply(value) * (1 - apply(value));
   }
 
 }