  We describe a scenario for large-scale magnetic field generation and particle
acceleration in a collisionless collision of cold plasma clouds. A
first-principle (i.e. using particles) numerical simulation of this process
might be possible. Our scenario is essentially 3D. We argue that {\it
large-scale} magnetic fields are not generated in 2D, even in collisionless
plasma.
  We calculate and numerically simulate magnetic field generation by
relativistic collisionless Kelvin-Helmholtz instability in 2D. Collisionless
tangential discontinuity might be more important than collisionless shock,
because tangential discontinuity remains unstable even in the hydro limit, when
the shock stabilizes.
