Error in Iwahori Hecke algebra for Coxeter Group with implementation='coxeter3'

[Jens-Erik Riedel]

In sage 10.7.beta8 and later I get an error when creating the Iwahori Hecke algebra for a Coxeter group if the Coxeter group has implementation='coxeter3' which is supplied by the optional "coxeter3" package.

I think that this error is introduced by PR #40328 which was merged into sage 10.7.beta8.

Steps to reproduce:

sage: W_cox3 = CoxeterGroup('A2', implementation='coxeter3'); W_cox3
sage: T_cox3 = IwahoriHeckeAlgebra(W_cox3, 1)

This results in a rather long error message in which the most important part, I think, is a pointer to line 512 of src/sage/algebras/iwahori_hecke_algebra.py where the attribute W.is_commutative is checked. The error messages ends with "AttributeError: 'CoxeterGroup_with_category' object has no attribute 'is_commutative'".

Cross check: This error does not occur in sage-10.7.beta7 or earlier. This error does also not occur if I replace W_cox3 by W_refl = CoxeterGroup('A2', implementation='reflection'), because W_refl has an attribute is_commutative but W_cox3 has not.

[Jens-Erik Riedel]

This error is fixed in PR #40666.