  This paper intends to survey the vast literature devoted to a problem posed
by Wilf in 1978 which, despite the attention it attracted, remains unsolved. As
it frequently happens with combinatorial problems, many researchers who got
involved in the search for a solution thought at some point that a solution
would be just around the corner, but in the present case that corner has never
been reached.
  By writing this paper I intend to give the reader a broad approach on the
problem and, when possible, connections between the various available results.
With the hope of gathering some more information than just using set inclusion,
at the end of the paper a slightly different way of comparing results is
developed.
