Currently the path to thebe.js is hard-coded in the form of a symlink relative to the doc source. Eventually one might want to be able to use this from an OS package instead, and not assume it's in $SAGE_LOCAL.

This can be viewed as a follow-up to #15105

Component: build

Author: Erik Bray

Branch/Commit: 9175036

Reviewer: Volker Braun

Issue created by migration from https://trac.sagemath.org/ticket/22061