Let (W,S)(W,S)(W,S) and (W′,S′)(W',S')(W′,S′) be Coxeter systems, with J⊆SJ\subseteq SJ⊆S and J′⊆S′J'\subseteq S'J′⊆S′. For u,v∈WJu,v\in W^Ju,v∈WJ and u′,v′∈(W′)J′u',v'\in (W')^{J'}u′,v′∈(W′)J′ with u⩽vu\leqslant vu⩽v and u′⩽v′u'\leqslant v'u′⩽v′, write ……
