Description: Define the class of extensions of RR . This is a shorthand for
listing the necessary conditions for a structure to admit a canonical
embedding of RR into it. Interestingly, this is not coming from a
mathematical reference, but was from the necessary conditions to build the
embedding at each step ( ZZ , QQ and RR ). It would be
interesting see if this is formally treated in the literature. See
isrrext for a better readable version. (Contributed by Thierry Arnoux, 2-May-2018)