Sentence:  It <WSD>was</WSD> `` Duty '' he said that his parents had given him as a rule - beyond even the love that suffused his being and the sense of humor with which he was largely supplied - and it was duty he would perform , though it cost him acute pain and exhausted him by the age of fifty . 
Meaning of the word marked with <WSD> tags:  be identical to; be someone or something