theorem
  ex x st sum(a,x,o)=o
proof
  consider x such that
A1: congr a,o,o,x by Th63;
A2: congr o,a,x,sum(a,x,o) by Def5;
  congr o,a,x,o by A1,Th69;
  hence thesis by A2,Th62;
end;
