Sentence:  `` I paint the nothing '' , he said <WSD>once</WSD> to Franz Kline and myself , `` the nothing that is behind the something , the inexpressible , unpaintable ' tick ' in the unconscious , the ' spirit ' of the moment resting forever , suspended like a huge balloon , in non time '' . 
Meaning of the word marked with <WSD> tags:  on one occasion