So the sentences in the type logic look like this, and they prove things are the form that some expression has some type, and they do that under the assumption that object identifiers have some types given by O, methods have signatures given by M, and the enclosing class, the current class in which E sits, in, in which we are doing our type checking, is class C. And the whole reason for this additional piece here we haven't actually discussed this before why we needed this C. It is because self-types meaning depends on the enclosing class. 