Doctest failure with nauty-gentreeg #34133
Description
with system nauty 2.6r7+ds-1 on debian-stretch (https://github.com/sagemath/sage/runs/7243040063?check_suite_focus=true)
The interface and doctest were added in #33670
Component: graph theory
Issue created by migration from https://trac.sagemath.org/ticket/34133