::You know Arthur, it's a good question. When I think of an "expression" I think it consists of one or more symbols. Logical symbols I think of as individual symbols. However every individual symbol can be thought of as an expression. So I am open minded as to how to organize it. However we should choose one way and stick to it. It is an interesting situation. Usually we want a subcategory to be entirely instances of it's category. In this case, it works both ways.