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The Weisfeiler–Leman (WL) hierarchy is a family of color-refinement procedures that iteratively color vertices (or k-tuples of vertices) to capture graph structure for isomorphism testing.
Graph isomorphism asks whether two graphs are the same up to renaming vertices; the Weisfeiler–Leman (WL) test is a powerful heuristic that often distinguishes non-isomorphic graphs quickly.