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).




The object closure is a method function.