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diff --git a/config-model/src/main/java/com/yahoo/searchdefinition/expressiontransforms/FunctionShadower.java b/config-model/src/main/java/com/yahoo/searchdefinition/expressiontransforms/FunctionShadower.java
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+++ b/config-model/src/main/java/com/yahoo/searchdefinition/expressiontransforms/FunctionShadower.java
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+// Copyright 2017 Yahoo Holdings. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+package com.yahoo.searchdefinition.expressiontransforms;
+
+import com.yahoo.searchdefinition.RankProfile;
+import com.yahoo.searchlib.rankingexpression.RankingExpression;
+import com.yahoo.searchlib.rankingexpression.rule.CompositeNode;
+import com.yahoo.searchlib.rankingexpression.rule.ExpressionNode;
+import com.yahoo.searchlib.rankingexpression.rule.FunctionNode;
+import com.yahoo.searchlib.rankingexpression.rule.ReferenceNode;
+import com.yahoo.searchlib.rankingexpression.transform.ExpressionTransformer;
+
+/**
+ * Transforms function nodes to reference nodes if a rank profile function shadows a built-in function.
+ * This has the effect of allowing rank profile functions to redefine built-in functions.
+ * Another effect is that we can add built-in functions over time
+ * without fear of breaking existing users' functions with the same name.
+ *
+ * However, there is a (largish) caveat. If a user has a function with a certain number
+ * of arguments, and we add in a built-in function with a different arity,
+ * this will cause parse errors as the Java parser gives precedence to
+ * built-in functions.
+ *
+ * @author lesters
+ */
+public class FunctionShadower extends ExpressionTransformer<RankProfileTransformContext> {
+
+    @Override
+    public RankingExpression transform(RankingExpression expression, RankProfileTransformContext context) {
+        String name = expression.getName();
+        ExpressionNode node = expression.getRoot();
+        ExpressionNode result = transform(node, context);
+        return new RankingExpression(name, result);
+    }
+
+    @Override
+    public ExpressionNode transform(ExpressionNode node, RankProfileTransformContext context) {
+        if (node instanceof FunctionNode)
+            return transformFunctionNode((FunctionNode) node, context);
+        if (node instanceof CompositeNode)
+            return transformChildren((CompositeNode)node, context);
+        return node;
+    }
+
+    private ExpressionNode transformFunctionNode(FunctionNode function, RankProfileTransformContext context) {
+        String name = function.getFunction().toString();
+        RankProfile.RankingExpressionFunction rankingExpressionFunction = context.rankProfile().getFunctions().get(name);
+        if (rankingExpressionFunction == null) {
+            return transformChildren(function, context);
+        }
+
+        int functionArity = function.getFunction().arity();
+        if (functionArity != rankingExpressionFunction.function().arguments().size())
+            return transformChildren(function, context);
+
+        ReferenceNode node = new ReferenceNode(name, function.children(), null);
+        return transformChildren(node, context);
+    }
+
+}