); however, at this point, we have to check (vii): we cannot choose ${{r_{\scriptscriptstyle{B}}}}$ and ${{f_{\scriptscriptstyle{B}}}}$ both freely since their sum has to be even. Now ${{r_{\scriptscriptstyle{K}}}}+ {{r_{\scriptscriptstyle{B}}}}+ {{f_{\scriptscriptstyle{K}}}}+ {{f_{\scriptscriptstyle{B}}}}= 0 + {{r_{\scriptscriptstyle{B}}}}+ 0 + {{f_{\scriptscriptstyle{B}}}}$ is odd (this parity depends only on 