reserve q,r,c,c1,c2,c3 for Quaternion;
reserve x1,x2,x3,x4,y1,y2,y3,y4 for Real;

theorem Th24:
  c1 * (c2 - c3) = c1 * c2 - c1 * c3
proof
  c1 * (c2 - c3)=c1 * c2 + c1 * (-c3) by Th17;
  hence thesis by Th21;
end;
