# File lib/clamp/attribute/instance.rb, line 63
      def default_from_environment
        return if self.defined?
        return if attribute.environment_variable.nil?
        return unless ENV.has_key?(attribute.environment_variable)
        # Set the parameter value if it's environment variable is present
        value = ENV[attribute.environment_variable]
        begin
          take(value)
        rescue ArgumentError => e
          command.send(:signal_usage_error, "$#{attribute.environment_variable}: #{e.message}")
        end
      end