  The elliptic flow in collisions of neutron-rich heavy-ion systems at
intermediate energies emerges as an observable sensitive to the strength of the
symmetry energy at supra-saturation densities. First results obtained by
comparing ratios or differences of neutron and hydrogen flows with predictions
of transport models favor a moderately soft to linear density dependence,
consistent with ab-initio nuclear-matter theories. Comprehensive data sets of
high accuracy can be expected to improve our knowledge of the equation of state
of asymmetric nuclear matter.
