|Description: Define plus infinity.
Note that the definition is arbitrary, requiring
only that be
a set not in and
(df-mnf 6847). We use to make it
independent of the
construction of ,
and Cantor's Theorem will show that it is
different from any member of and therefore . See pnfnre 6855,
mnfnre 6856, and pnfnemnf 6895.
A simpler possibility is to define as and as
, but that approach requires the Axiom of
Regularity to show
that and are different from each
other and from all
members of .