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- (
**forall**`x`:`y`<=`x`)`x`is bound,`y`is free; the formula is therefore*not closed*.- true in assignment [
`y`|->`0`] over the domain of natural numbers with the usual interpretation of ` <= `, because 0 <=`x`is true for every natural number`x`:0 <= 0, 0 <= 1, 0 <= 2, ...

- (
**exists**`x`: x | 15)`x`is bound; the formula is*closed*.- true for every assignment over the natural numbers with `|'
interpreted as `divides', because
`x`| 15 is true for some natural number`x`(e.g. for 3):3 | 15.

Author: Wolfgang Schreiner

Last Modification: October 6, 1999