[en] Collisions of high-energy heavy ions are usually treated in terms of Fermi seas colliding with Fermi seas. Whereas the bulk of the cross section may be describable in this way, important quantum effects enter into peripheral processes which can be coherent from the standpoint of the nucleons in the target and/or those in the projectile. The longest-range part of the nucleon--nucleon force is spin- and isospin-dependent, so that such peripheral processes are well suited for pion production. Whereas 100 MeV/nucleon would not be sufficient for pion production in nucleon--nucleon collisions, coherent production should be appreciable. 2 figures