Some integer constants

This file contains definitions for the first integers, in order to improve the readability of statements and proofs:
Definition one:=(S O).
Definition two:=(S (S O)).
Definition three:=(S (S (S O))).
Definition four:=(S (S (S (S O)))).
These abbreviations were useful for shortening some commands, but necessited a lot of Unfold and Unfold ... in commands. We are not sure to maintain these definitions in the future versions of this work.