Prove Proposition 4.5.2 in Bourbaki about life map to monoid hom for a coxeter system

[jiajunma]

coxeter/Coxeter/StrongExchange.lean

Line 48 in c4f1e53

def mapLift {f : S → M} (h : ∀ s s', (f s * f s')^orderOf ((s :G) * s') = 1) :

One may need to develop some basic properties for dihedral subgroup (S4.2 Bourbaki) and state and prove Prop 5.4