Sentence:  Not only <WSD>is</WSD> Mr. Frelinghuysen a member of the House Foreign Affairs Committee , but he is the grandson of the man who was instrumental in opening relations between the United States and Korea , Frederick T. Frelinghuysen , secretary of state in the administration of Chester A. Arthur . 
Meaning of the word marked with <WSD> tags:  be identical to; be someone or something