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
  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;
