 \kangaroo \killer-whale\moose \wolf \narwhal\begin{array} { c } { \Phi ( \tau , \xi ) = - v \; \; \mathrm { f o r } \; \; \xi \leq - \xi _ { 1 } , } \\ { \Phi ( \tau , \xi ) = + v \; \; \mathrm { f o r } \; \; \xi \geq \xi _ { 0 } . } \\ \end{array}