The first example of formal mathematics that we shall consider is type theory itself. Since this is the ``primitive language'' of Nuprl it will not be necessary to write definitions; the basic constructors and atomic types are already available.
As we have already mentioned, the three atomic types are int, atom and void, and the types are classified into a cumulative hierarchy of universes, , , . A type in universe is said to be at level i. The atomic types all exist at level 1, i.e., in . We can express this for the type of integers with the statement int in U1. We now illustrate the mechanics of stating theorems one more time using this theorem as an example. Figure shows actual snapshots of the screen as the theorem is produced. First, a position is created in the library using the command cr t1 thm bot. The proof editor is entered using v t1, and the text editor is entered by positioning the cursor on <main proof goal> and hitting SEL . We now type int in U1 in the text editor and then hit .
Figure: Stating a Simple Theorem