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- Difference of two numbers is always is well-defined.
- Definition on top of
**N**.**Z**subset**N**x**N**.**N**not subset**Z**!

- We will see later how to "embed"
**N**into**Z**. - We will see later a "better" construction of
**Z**.

Author: Wolfgang Schreiner

Last Modification: November 16, 1999