fact: Definition braid_aba := fun s : seq 'I_n => match s with | [:: a; b; c] => if (a == c) && ((a.+1 == b) || (b.+1 == a)) then [:: [:: b; a; b]] else [::] | _ => [::] end.
imports: HB(structures) mathcomp(all_ssreflect) mathcomp(fingroup perm morphism presentation) mathcomp(ssralg poly ssrint) permcomp tools permuted combclass congr