premise: in the phrase “a billion dollars is a relatively small sum to spend on cancer research” the <mask> of small by relatively is needed to make the sentence accurate, rather than ludicrous
hypothesis: the best context for <mask> here is The alteration of the meaning of a word or phrase by another word or phrase; -- usually a restriction of the scope of the word modified; but not The act of modifying, or the state of being modified; a change.