theorem Th19:
  Fracadd(x,Fracadd(y,z)) = Fracadd(Fracadd(x,y),z)
  proof
    Fracadd(Fracadd(x,y),z)
     = [(x`1*y`2)*z`2 + (y`1*x`2)*z`2 + z`1*(x`2*y`2),(x`2*y`2)*z`2]
       by VECTSP_1:def 7
    .= [x`1*(y`2*z`2) + (y`1*x`2)*z`2 + z`1*(x`2*y`2),(x`2*y`2)*z`2]
       by GROUP_1:def 3
    .= [x`1*(y`2*z`2) + (y`1*z`2)*x`2 + z`1*(y`2*x`2),(x`2*y`2)*z`2]
       by GROUP_1:def 3
    .= [x`1*(y`2*z`2) + (y`1*z`2)*x`2 + (z`1*y`2)*x`2,(x`2*y`2)*z`2]
       by GROUP_1:def 3
    .= [x`1*(y`2*z`2) + (y`1*z`2)*x`2 + (z`1*y`2)*x`2,x`2*(y`2*z`2)]
       by GROUP_1:def 3
    .= [x`1*(y`2*z`2) + ((y`1*z`2)*x`2 + (z`1*y`2)*x`2),x`2*(y`2*z`2)]
       by RLVECT_1:def 3
    .= Fracadd(x,Fracadd(y,z)) by VECTSP_1:def 7;
    hence thesis;
  end;
