brew updateしたらエラーが出た。
こちらを参考に対処。
$ brew update
error: The following untracked working tree files would be overwritten by merge:
Library/Formula/py3cairo.rb
Please move or remove them before you can merge.
Aborting
Error: Failure while executing: git pull -q origin refs/heads/master:refs/remotes/origin/master
こちらを参考に対処。
$ brew tap --repairというメモ。
$ cd $(brew --prefix) && git fetch && git reset --hard origin/master
$ brew update