  Accurate particle simulations are essential for the next generation of
experiments in astroparticle physics. The Monte Carlo simulation library
PROPOSAL is a flexible tool to efficiently propagate high-energy leptons and
photons through large volumes of media, for example in the context of
underground observatories. It is written as a C++ library, including a Python
interface. In this paper, the most recent updates of PROPOSAL are described,
including the addition of electron, positron, and photon propagation, for which
new interaction types have been implemented. This allows the usage of PROPOSAL
to simulate electromagnetic particle cascades, for example in the context of
air shower simulations. The precision of the propagation has been improved by
including rare interaction processes, new photonuclear parametrizations,
deflections in stochastic interactions, and the possibility of propagating in
inhomogeneous density distributions. Additional technical improvements
regarding the interpolation routine and the propagation algorithm are
described.
