closure(M, S)
The closure of a subset S of a matroid (E, B) is the set cl(S) := {x $\in$ E : rank(S) = rank(S $\cup \{x\}$) }. The closure operator 2^E -> 2^E, S $\mapsto$ cl(S), completely determines the matroid (indeed, the maximal proper closed sets - i.e. hyperplanes - already determine the matroid).
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The object closure is a method function.
The source of this document is in /build/reproducible-path/macaulay2-1.25.06+ds/M2/Macaulay2/packages/Matroids/doc-Matroids.m2:1061:0.