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

theorem Th7:
  c1 = c1 + c2 - c2
proof
  thus c1 + c2 - c2 = c1 + (c2 - c2) by Th2
    .= c1 + 0q by QUATERNI:def 8
    .= c1 by Th3;
end;
