theorem Th16:
  a is right_complementable & b is right_complementable implies
  <%a,b%> is right_complementable
  proof
    given x being Element of N such that
A1: a+x = 0.N;
    given y being Element of N such that
A2: b+y = 0.N;
    take <%x,y%>;
    thus <%a,b%>+<%x,y%> = <%a+x,b+y%> by Def9
    .= 0.Cayley-Dickson(N) by A1,A2,Def9;
  end;
