formula: require instances to use a subclass
formula: require instances to use a subclass

nandub/brew,JCount/brew,JCount/brew,JCount/brew,nandub/brew,Homebrew/brew,MikeMcQuaid/brew,JCount/brew,MikeMcQuaid/brew,nandub/brew,Homebrew/brew,MikeMcQuaid/brew,Homebrew/brew,Homebrew/brew,nandub/brew,MikeMcQuaid/brew