theorem
  a,b // opposite(a,o),opposite(b,o)
proof
  sum(b,opposite(b,o),o)=o by Def6;
  then congr o,b,opposite(b,o),o by Def5;
  then
A1: congr b,o,o,opposite(b,o) by Th69;
  sum(a,opposite(a,o),o)=o by Def6;
  then congr o,a,opposite(a,o),o by Def5;
  then congr a,o,o,opposite(a,o) by Th69;
  hence thesis by A1,Th73;
end;
