Sentence:  Through 1911 and 1912 , as the Cubist facet-plane 's tendency to adhere to the literal surface became harder and <WSD>harder</WSD> to deny , the task of keeping the surface at arm's length fell all the more to eye-undeceiving contrivances . 
Meaning of the word marked with <WSD> tags:  not easy; requiring great physical or mental effort to accomplish or comprehend or endure