let z, x1, x2, Y be set ; :: thesis: ( z in [:{x1,x2},Y:] implies ( ( z `1 = x1 or z `1 = x2 ) & z `2 in Y ) )
assume z in [:{x1,x2},Y:] ; :: thesis: ( ( z `1 = x1 or z `1 = x2 ) & z `2 in Y )
then ( z `1 in {x1,x2} & z `2 in Y ) by Th10;
hence ( ( z `1 = x1 or z `1 = x2 ) & z `2 in Y ) by TARSKI:def 2; :: thesis: verum