Sentence:  The great majority of present-day linguists fall into one or more of a number of overlapping types : those who are convinced that tone cannot be analysed , those who are personally scared of tone and tone languages generally , those who are convinced that tone <WSD>is</WSD> merely an unnecessary marginal feature in those languages where it occurs , those who have no idea how to proceed with tone analysis , those who take a simplistic view of the whole matter . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)