 if \(\Gamma , \phi \vdash_{\bL } \psi\) and \(\Gamma , \psi \vdash_{\bL } \phi\), then \(\Gamma , \delta(p/\phi) \vdash_{\bL } \delta(p/\psi)\) and \(\Gamma , \delta(p/\psi) \vdash_{\bL } \delta(p/\phi)\),