A parabolic subgroup is represented by the set of indices of generating reflections.
The object Parabolic is a type, with ancestor classes Set < Tally < VirtualTally < HashTable < Thing.
The source of this document is in /build/reproducible-path/macaulay2-1.25.05+ds/M2/Macaulay2/packages/WeylGroups.m2:3311:0.