reserve x for set,
  i,j,k,n for Nat,
  K for Field;

theorem Th1:
  for a, A being set st a in A holds {a} in Fin A
proof
  let a, A be set;
  assume a in A;
  then {a} c= A by ZFMISC_1:31;
  hence thesis by FINSUB_1:def 5;
end;
