module Roda::RodaPlugins

def self.load_plugin(name)

not register itself correctly.
plugin doesn't exist, or a RodaError if it exists but it does
require it and return it. This raises a LoadError if such a
If the registered plugin already exists, use it. Otherwise, 
def self.load_plugin(name)
  h = @plugins
  unless plugin = h[name]
    require "roda/plugins/#{name}"
    raise RodaError, "Plugin #{name} did not register itself correctly in Roda::RodaPlugins" unless plugin = h[name]
  end
  plugin
end