@Override
    public boolean equals(Object o) {
        if (o == this) return true;
        if (o.hashCode() != this.hashCode()) return false; // because this has a fast hashCode
        if ( ! (o instanceof Reference)) return false;
        Reference other = (Reference) o;
        if (!Objects.equals(other.name(), this.name())) return false;
        if (!Objects.equals(other.arguments, this.arguments)) return false;
        if (!Objects.equals(other.output, this.output)) return false;
        if (!Objects.equals(other.isIdentifier, this.isIdentifier)) return false;
        return true;
    }
If the arguments of this contains a single argument which is an identifier, it is returned.
Otherwise null is returned.
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/Reference.java