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*Definition:*
If `x` is an element of `S` such that no element of `S` is
smaller than `x`, then `x` is called a
*minimal element* of `S`.

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

If `x` is an element of `S` such that no
element of `S` is
greater than `x`, then `x` is called a
*maximal element* of `S`.

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

*Minimal and maximal elements are not necessarily unique.*

Author: Wolfgang Schreiner

Last Modification: January 18, 2000