While exploring hexo, I ran into a problem installing it.
A quick check of yarn's version number showed that I was somehow way out of date.
yarn's installation directions
was merely a matter of performing a
yarn also claimed it wasn't able to do, or to install an
pubkey.gpg and install/update with
These solutions didn't work either.
The problem was that another package,
cmdtest, had it's own
obsolete copy of yarn.
The solution was simply to uninstall it, then install yarn with apt:
sudo apt remove cmdtest
curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add -
echo "deb https://dl.yarnpkg.com/debian/ stable main" | sudo tee /etc/apt/sources.list.d/yarn.list
sudo apt-get update && sudo apt-get install yarn