Sentence:  Then in 2 we show that any line involution with the properties that ( a ) It has no complex of invariant lines , and ( b ) Its singular lines form a complex consisting exclusively of the lines which meet a twisted curve , <WSD>is</WSD> necessarily of the type discussed in 1 . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)