theorem Th58: not y in proj2 X implies [:A,{y}:] misses X
proof
set X2=proj2 X, Y=[:A,{y}:], Z=X/\Y; assume A1: not y in X2; assume
Y meets X; then Z<>{};
then consider z being object such that
A2: z in Z; set x1=z`1, y1=z`2;
x1 in A & y1 in {y} & z=[x1,y1] & z in X by A2, MCART_1:10,21; then
y1=y & y1 in X2 by TARSKI:def 1, XTUPLE_0:def 13;
hence contradiction by A1;
end;
