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 Th78:
  sum(a,b,o)=sum(b,a,o)
proof
  congr o,b,a,sum(b,a,o) by Def5;
  then congr o,a,b,sum(b,a,o) by Th69;
  hence thesis by Def5;
end;
