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 Th70:
  congr a,b,p,q & congr b,c,q,s implies congr a,c,p,s
proof
  assume congr a,b,p,q & congr b,c,q,s;
  then congr b,q,a,p & congr b,q,c,s by Th69;
  then congr a,p,c,s by Th65;
  hence thesis by Th68;
end;
