Björner's EL-Labelling of the Partition Lattice Combinatorics Szemerédi--Trotter Theorem Combinatorics Products Preserve Cohen-Macaulayness of Finite Graded Posets Combinatorics Difference Set Parameter Equation Combinatorics Augmentation Lemma for Network Flows Combinatorics Countability of Finite Subsets Discrete Mathematics Rank Axioms for Matroid Rank Functions Discrete Mathematics Diagonal Intersection of Clubs Logic Discrete Mathematics Area Combinatorics Subarea