class Roda::RodaPlugins::Middleware::Forwarder

Forward instances are what is actually used as middleware.

def call(env)

pass handling of the request to the next middleware.
If this returns a result, return that result directly. Otherwise,
When calling the middleware, first call the current middleware. 
def call(env)
  res = nil
  call_next = catch(:next) do
    res = @mid.call(env)
    false
  end
  if call_next
    @app.call(env)
  else
    res
  end
end

def initialize(mid, app)

Store the current middleware and the next middleware to call. 
def initialize(mid, app)
  @mid = mid.app
  @app = app
end