  In this letter, I have considered one-dimensional quantum system with
different masses $m$ and $M$, which does not appear integrable in general.
However I have found an exact two-body wave function and due to the extension
of the integrable system to more general system, it was concluded that the
rapidity or quasi-momentum in the integrable system should be regarded as a
modification of velocity rather than that of momentum. I have also considered
the three-body wave function and argued its integrable condition.
