  Stochastic pumps are models of artificial molecular machines which are driven
by periodic time variation of parameters, such as site and barrier energies.
The no-pumping theorem states that no directed motion is generated by variation
of only site or barrier energies [S. Rahav, J. Horowitz, and C. Jarzynski,
Phys. Rev. Lett., 101, 140602 (2008)]. We study stochastic pumps of several
interacting particles and demonstrate that the net current of particles satisfy
an additional no- pumping theorem.
