Sentence:  Leaving the theatre after the performance , I had a flash of intuition that life , after all ( as Rilke said ) , is just a search for the nonexistent cup of hot coffee , and that this unpretentious , moving , clever , bitter slice of life <WSD>was</WSD> the greatest thing to happen to the American theatre since Brooks Atkinson retired . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)