  After reviewing some basic properties of RT algebras, which appear to be the
natural framework to deal with integrable systems in presence of an impurity,
we show how any integrable system (including these possessing translation
invariance) can be promoted to an integrable system with an impurity which can
reflect and transmit particles. The technics allows bulk translation invariant
$S$-matrices while avoiding the no-go theorem stated recently about these
laters.
  Presented at the Vth International workshop on Lie theory and its
applications in physics, Varna (Bulgaria), June 16-22, 2003
