def define_config(name, &blk)
      if @definitions.has_key?(name)
        raise ConfigError.new("#{name.to_s.upcase} is already registered")
      end
      @definitions[name] = Proc.new do |env|
        value = yield env
        if value.kind_of?(Hash)
          value
        else
          {name => value}
        end
      end
    end
def define_config(name, &blk)
      if @definitions.has_key?(name)
        raise ConfigError.new("#{name.to_s.upcase} is already registered")
      end
      @definitions[name] = Proc.new do |env|
        value = yield env
        if value.kind_of?(Hash)
          value
        else
          {name => value}
        end
      end
    end
Define a handler for a configuration value.