public RankingExpressionFunction findFunction(String name) {
        RankingExpressionFunction function = functions.get(name);
        if (function != null) return function;
        return uniquelyInherited(p -> p.findFunction(name), "function '" + name + "'").orElse(null);
    }
Returns the inputs of this, which also includes all inputs of the parents of this.
config-model/src/main/java/com/yahoo/schema/RankProfile.java