  The workflow satisfiability problem (WSP) is a well-studied problem in access
control seeking allocation of authorised users to every step of the workflow,
subject to workflow specification constraints. It was noticed that the number
$k$ of steps is typically small compared to the number of users in the
real-world instances of WSP; therefore $k$ is considered as the parameter in
WSP parametrised complexity research. While WSP in general was shown to be
W[1]-hard, WSP restricted to a special case of user-independent (UI)
constraints is fixed-parameter tractable (FPT). However, restriction to the UI
constraints might be impractical.
  To efficiently handle non-UI constraints, we introduce the notion of
branching factor of a constraint. As long as the branching factors of the
constraints are relatively small and the number of non-UI constraints is
reasonable, WSP can be solved in FPT time.
  Extending the results from Karapetyan et al. (2019), we demonstrate that
general-purpose solvers are capable of achieving FPT-like performance on WSP
with arbitrary constraints when used with appropriate formulations. This
enables one to tackle most of practical WSP instances. While important on its
own, we hope that this result will also motivate researchers to look for
FPT-aware formulations of other FPT problems.
