theorem Th9:
 X c= Y implies proj2 X c= proj2 Y
proof assume
A1: X c= Y;
 let x be object;
 assume x in proj2 X;
  then consider y such that
A2: [y,x] in X by Def13;
  [y,x] in Y by A1,A2;
 hence thesis by Def13;
end;
