年が代わりましたのでお作法として
brew doctor
したところ、out of dateというありがたいお告げを受けましたのですかさず
brew update
しかし、その後のbrew doctorでWarningが。
Warning: You have external commands with conflicting names.
Found command `brew-cask` in following places:
/usr/local/bin/brew-cask
/usr/local/Library/Taps/caskroom/homebrew-cask/cmd/brew-cask.rb
調べてみると、どうやらHomebrew-CaskはHomwbrewに一体化された模様
https://github.com/caskroom/homebrew-cask
の冒頭から引用
Homebrew-Cask
Important December update: Homebrew-Cask will now be kept up to date together with Homebrew (see #15381 for details). If you haven’t yet, run brew uninstall --force brew-cask; brew update to switch to the new system.
Important: At some point in the future Homebrew-cask will change its behaviour from linking apps to moving them. See issue #13201 for details.
そんなわけで指定されたコマンドを実行
brew uninstall --force brew-cask; brew update
これで無事Warningが出なくなりました。