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 Th22;
  hence thesis;
end;
