  Scheduling with assignment restrictions is an important special case of
scheduling unrelated machines which has attracted much attention in the recent
past. While a lower bound on approximability of 3/2 is known for its most
general setting, subclasses of the problem admit polynomial-time approximation
schemes. This note provides a PTAS for tree-like hierarchical structures,
improving on a recent 4/3-approximation by Huo and Leung.
