reserve a,x,y for object, A,B for set,
  l,m,n for Nat;

theorem
  A* c= B* implies A c= B
proof
  assume
A1: A* c= B*;
  let x be object;
  assume x in A;
  then <*x*> in A* by Th18;
  hence thesis by A1,Th18;
end;
