CSPLib: Twenty Years On
  In 1999, we introduced CSPLib, a benchmark library for the constraints
community. Our CP-1999 poster paper about CSPLib discussed the advantages and
disadvantages of building such a library. Unlike some other domains such as
theorem proving, or machine learning, representation was then and remains today
a major issue in the success or failure to solve problems. Benchmarks in CSPLib
are therefore specified in natural language as this allows users to find good
representations for themselves. The community responded positively and CSPLib
has become a valuable resource but, as we discuss here, we cannot rest.
