  This short document illustrates QLUSTER: a toy model for populations of
binary black holes in dense astrophysical environments. QLUSTER is a simple
tool to investigate the occurrence and properties of hierarchical black-hole
mergers detectable by gravitational-wave interferometers. QLUSTER is not meant
to rival the complexity of state-of-the-art population synthesis and N-body
codes but rather provide a fast, approximate, and easy-to-interpret framework
to investigate some of the key ingredients of the problem. These include the
binary pairing probability, the escape speed of the host environment, and the
merger generation. We also introduce the "hierarchical-merger efficiency" -- an
estimator that quantifies the relevance of hierarchical black-hole mergers in a
given astrophysical environment.
