Sentence:  But if no two lines of the regulus of multiple secants of \ g can intersect , then the regulus must <WSD>be</WSD> quadratic , or in other words , \ g must be either a * * f or a * * f curve on a nonsingular quadric surface . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)