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*Definition:*
If `x` is an element of `S` such that `x` is less than or equal
to any element of `S`, then `x` is called the
*least element* of `S`:

xis least element ofSw.r.t.<=: <=>xinS/\forallyinS:x<=y.

If `x` is an element of `S` such that
`x` is greater than or equal
to any element of `S`, then
`x` is called the *greatest element* of `S`:

xis greatest element ofSw.r.t.<=: <=>xinS/\forallyinS:y<=x.

*Least respectively greatest element is unique (if it exists).*

Author: Wolfgang Schreiner

Last Modification: January 18, 2000