reserve x,y,z,X,Y for set;
reserve X,Y for non empty set,
  f for Function of X,Y;

theorem Th6:
  for B being Element of Fin X for x st x in B holds x is Element of X
proof
  let B be Element of Fin X;
A1: B c= X by FINSUB_1:def 5;
  let x;
  assume x in B;
  hence thesis by A1;
end;
