on the page:
https://www.gnu.org/software/emacs/download.html

brew cask install emacs

should be replaced with:

brew install --cask emacs

as the cask subcommand has been removed from homebrew, in favor of a --cask flag.

relevant commit removing the cask subcommand from the usage instructions on the official homebrew github:
https://github.com/Homebrew/homebrew-cask/commit/1fb821a4a50f75c92afe1ded44ccc6709314461b#diff-58a825c70bccfbe691d7ddbddb133275f4f657de6aeb4a502fa9fce279f8bf8d

tyler@iheartapis.com
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