theorem Th8:
 X c= Y implies proj1 X c= proj1 Y
proof assume
A1: X c= Y;
 let x be object;
 assume x in proj1 X;
  then consider y such that
A2: [x,y] in X by Def12;
  [x,y] in Y by A1,A2;
 hence thesis by Def12;
end;
