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*Definition:* `z` is a *difference* of
`x` and `y` if `x` = `y`+`z`.

x-y:=suchz:x=z+y.

- Difference is not defined for every
`x`and`y`:There is no

`z`with`1`=`z`+`2`, thus 1-2 is undefined. - If a difference exists, it is unique:
**forall**`x`,`y`,`z`_{0},`z`_{1}: (`x`=`z`_{0}+`y`/\`x`=`z`_{1}+`y`) =>`z`_{0}=`z`_{1}. - If
`x`>=`y`, the difference of`x`and`y`is defined:**forall**`x`,`y`:`x`>=`y`=>`x`= (`x`-`y`)+`y`.

Author: Wolfgang Schreiner

Last Modification: November 16, 1999