@Override
    protected final Expression doConvert(Expression exp) {
        if (duplicate) {
            duplicate = false;
            return null;
        }
        transformed = true;
        return new StatementExpression(newTransform(), exp);
    }
Inserts a "newTransform()" before expressions that "requiresTransform()"

@author Simon Thoresen Hult
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/ValueTransformProvider.java