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partial ordering

(i.e. it is reflexive (x R x) and transitive (x R y R z =@#

x R z)) and it is also antisymmetric (x R y R x =@# x = y).

The ordering is partial, rather than total, because there may

exist elements x and y for which neither x R y nor y R x.

In domain theory, if D is a set of values including the

undefined value (bottom) then we can define a partial

ordering relation #@= on D by

x #@= y if x = bottom or x = y.

The constructed set D x D contains the very undefined element,

(bottom, bottom) and the not so undefined elements, (x,

bottom) and (bottom, x). The partial ordering on D x D is

then

(x1,y1) #@= (x2,y2) if x1 #@= x2 and y1 = y2.

The partial ordering on D - D is defined by

f #@= g if f(x) #@= g(x) for all x in D.

(No f x is more defined than g x.)

A lattice is a partial ordering where all finite subsets

have a least upper bound and a greatest lower bound.