reserve SAS for Semi_Affine_Space;
reserve a,a9,a1,a2,a3,a4,b,b9,c,c9,d,d9,d1,d2,o,p,p1,p2,q,r,r1,r2,s,x, y,t,z
  for Element of SAS;

theorem Th81:
  congr a,o,o,opposite(a,o)
proof
  sum(a,opposite(a,o),o)=o & congr o,a,opposite(a,o),sum(a,opposite(a,o),o
  ) by Def5,Def6;
  hence thesis by Th69;
end;
