A hyperplane is an affine-linear subspace of codimension one. An arrangement is a finite set of hyperplanes.
The object Arrangement is a type, with ancestor classes HashTable < Thing.
The source of this document is in /build/reproducible-path/macaulay2-1.25.05+ds/M2/Macaulay2/packages/HyperplaneArrangements.m2:1087:0.