  Let $L$ be a planar semimodular lattice. We call $L$ \emph{slim}, if it has
no $\mthree$ sublattice. Let us define an \emph{SPS lattice} as a slim, planar,
semimodular lattice $L$.
  In 2016, I proved a property of congruences of SPS lattices (Two-cover
Property) and raised the problem of characterizing them.
  Since then, more than 50 papers have been published contributing to this
problem. In this survey, I provide an overview of this field with major
contributions by G\'abor Cz\'edli.
