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 Th64:
  congr a,b,a,b
proof
  now
    assume a<>b;
    then consider p,q such that
A1: parallelogram a,b,p,q by Lm1;
    parallelogram p,q,a,b by A1,Th43;
    hence thesis;
  end;
  hence thesis;
end;
