  The PUMAS library is a transport engine for muon and tau leptons in matter.
It can operate with a configurable level of details, from a fast deterministic
CSDA mode to a detailed Monte~Carlo simulation. A peculiarity of PUMAS is that
it is revertible, i.e. it can run in forward or in backward mode. Thus, the
PUMAS library is particularly well suited for muography applications. In the
present document, we provide a detailed description of PUMAS, of its physics
and of its implementation.
