  State-of-the-art solvers for symmetry detection in combinatorial objects are
becoming increasingly sophisticated software libraries. Most of the solvers
were initially designed with inputs from combinatorics in mind (nauty, bliss,
Traces, dejavu). They excel at dealing with a complicated core of the input.
Others focus on practical instances that exhibit sparsity. They excel at
dealing with comparatively easy but extremely large substructures of the input
(saucy). In practice, these differences manifest in significantly diverging
performances on different types of graph classes.
  We engineer a preprocessor for symmetry detection. The result is a tool
designed to shrink sparse, large substructures of the input graph. On most of
the practical instances, the overall running time improves significantly for
many of the state-of-the-art solvers. At the same time, our benchmarks show
that the additional overhead is negligible.
  Overall we obtain single algorithms with competitive performance across all
benchmark graphs. As such the preprocessor bridges the disparity between
solvers that focus on combinatorial graphs and large practical graphs. In fact,
on most of the practical instances the combined setup significantly outperforms
previous state-of-the-art.
