  We propose and demonstrate that a gamma-gamma collider with W_gg < 12 GeV can
be added to the European XFEL with a minimal disruption to its main program.
High-energy photons will be obtained by Compton scattering of 0.5 micron laser
photons on the existing 17.5 GeV electron beams. Such a gamma-gamma collider
would be an excellent place for the development and application of modern
technologies: powerful lasers, optical cavities, superconducting linacs, and
low-emittance electron sources -- as well as training the next generation of
accelerator physicists and engineers. The physics program would include
spectroscopy of C=+ resonances in various J^P states b\bar{b}, c\bar{c},
four-quark states, quark molecules and other exotica) in a mass range barely
scratched by past and not covered by any current or planned experiments.
Variable circular and linear polarizations will help in the determination of
quantum numbers and measurement of polarization components of the gamma-gamma
cross section (\sigma_\perp, \sigma_\parallel, \sigma_0, \sigma_2).
