class T::Private::Methods::DeclBuilder

def abstract
  check_live!
  case decl.mode
  when Modes.standard
    decl.mode = Modes.abstract
  when Modes.abstract
    raise BuilderError.new(".abstract cannot be repeated in a single signature")
  else
    raise BuilderError.new("`.abstract` cannot be combined with any of `.override`, `.implementation`, or "\
          "`.overridable`.")
  end
  self
end