$\mathrm{env}\mskip-1.0mu\overline{U}\left(-c,x\right)=\begin{cases}\left({U}^{%
2}\left(-c,x\right)+{\overline{U}}^{2}\left(-c,x\right)\right)^{1/2},&0\leq x%
\leq X_{c},\\
\sqrt{2}\ \overline{U}\left(-c,x\right),&X_{c}\leq x<\infty,\end{cases}$