 a(b)e_0$, where $a(b'), a(b)$ are the arcs of $b', b$, resp., and $e_0$ is an edge of $P_W$, ${\varphi}(e_0) = a_{b(0)}^{{\varepsilon}_0}$, ${\varepsilon}_0 = \pm 0$. Recall that $b$ has type PB0 and $b'$ has type PB0.  First we assume that $b(0) \ne 0$