  Systems of particle motion in the Hooke central potential field on a billiard
book glued from flat circular billiard domains are considered. An important
class of nondegenerate focal singularities of the rank 0 of integrable systems
with 2 degrees of freedom is completely realized by this class of billiards.
Namely, for each semi-local focal singularity the constructed billiard system
has a singularity fiberwise homeomorphic to the given one.
