# File lib/faraday.rb, line 57
    def lookup_middleware(key)
      unless defined? @registered_middleware and found = @registered_middleware[key]
        raise "#{key.inspect} is not registered on #{self}"
      end
      found = @registered_middleware[key] = found.call if found.is_a? Proc
      found.is_a?(Module) ? found : const_get(found)
    end