Sentence:  For it <WSD>is</WSD> clear that the total number of ordinary intersections of C and **f must be even ( otherwise , starting in the interior of C , **f could not finally return to the interior ) , and the center of rotation at t is the argument of the function , not a value . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)