public Value evaluate(Context context) {
        return root.evaluate(context);
    }
Returns the value of evaluating this expression over the given context.

@param context The variable bindings to use for this evaluation.
@return the evaluation result.
@throws IllegalArgumentException if there are variables which are not bound in the given map
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/RankingExpression.java