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

theorem
  - c1.|.c2 = c1.|.(-c2)
proof
  c1.|.(-c2)=c1 * -(c2*') by QUATERNI:55
    .=-(c1 * c2*') by Th21;
  hence thesis;
end;
