Mystical secrets of the bookworm
∵ unmerged /usr is unsupported in bookworm and sid has been feeding bookworm since 2021-08-14,
unmerged /usr is also unsupported in sid since 2021-08-14,
no one using any portion of either bookworm or sid since 2021-08-14 should have any expectation that things should function correctly with unmerged /usr ,
∴ anyone using any portion of either bookworm or sid should execute
apt install usrmerge or perform its equivalent on or prior to 2021-08-14.