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 Th73:
  congr b,a,p,q & congr c,a,p,r implies b,c // q,r
proof
  assume congr b,a,p,q & congr c,a,p,r;
  then congr b,c,r,q by Th71;
  then b,c // r,q by Th57;
  hence thesis by Th6;
end;
