Sentence:  Moreover , if * * f and * * f <WSD>are</WSD> two planes intersecting in a line l , tangent to Q at a point P , the two free intersections of the image curves * * f and * * f must coincide at P ' , the image of P , and at this point * * f and * * f must have a common tangent l ' . 
Meaning of the word marked with <WSD> tags:  be identical to; be someone or something