- Section 6, the definition of R_{st}^{T_r} and Lemma 7

Wrong:
s \in V(\overrightarrow{T_r}) \land t \in V(\overleftarrow{T_r})

Correct:
s \in V(\overleftarrow{T_r}) \land t \in V(\overrightarrow{T_r})
(it means that r is reachable from s and t is reachable from r.)