railties/lib/rails/generators/rails/plugin/templates/lib/%namespaced_name%/engine.rb.tt

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<%= wrap_in_modules <<~rb
  class Engine < ::Rails::Engine
  #{mountable? ? "  isolate_namespace " + camelized_modules : " "}
  #{api? ? "  config.generators.api_only = true" : " "}
  end
rb
%>