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
  sum(a,a,o)=o implies a=o
proof
  assume sum(a,a,o)=o;
  then congr o,a,a,o by Def5;
  hence thesis by Th56;
end;
