public RankingExpression transform(RankingExpression expression, CONTEXT context) {
        return new RankingExpression(expression.getName(), transform(expression.getRoot(), context));
    }
Superclass of expression transformers. The scope (lifetime) of a transformer instance is a single compilation
of all the expressions in one rank profile.

@author bratseth
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/transform/ExpressionTransformer.java