theorem Th20:
  Al is countable implies
  ExCl(Al) is non empty & ExCl(Al) is countable
proof
  assume A1: Al is countable;
  set x =the  bound_QC-variable of Al,p =the  Element of CQC-WFF(Al);
  set a = Ex(x,p);
  a in ExCl(Al) by Def3;
  hence ExCl(Al) is non empty;
  CQC-WFF(Al) is countable by Th19,A1;
  hence thesis;
end;
