public EqualsOnlyAttributeValueList this[Rule rule] => this[null, rule, false];
public EqualsOnlyAttributeValueList this[string attr, Rule rule] => this[attr, rule, false];
public EqualsOnlyAttributeValueList this[Rule rule, bool isDn] => this[null, rule, isDn];
public EqualsOnlyAttributeValueList this[string attr, Rule rule, bool isDn] => this.Attributes.ContainsKey(attr) ? this.Attributes[attr] : null;