def guard(reason = nil, &block)
      raise ArgumentError, 'must be called with a block' unless block_given?
      callback do |*callback_args|
        begin
          unless block.call(*callback_args)
            raise ::DeferrableGratification::GuardFailed.new(reason, callback_args)
          end
        rescue => exception
          fail(exception)
        end
      end
      self
    end
def guard(reason = nil, &block)
      raise ArgumentError, 'must be called with a block' unless block_given?
      callback do |*callback_args|
        begin
          unless block.call(*callback_args)
            raise ::DeferrableGratification::GuardFailed.new(reason, callback_args)
          end
        rescue => exception
          fail(exception)
        end
      end
      self
    end
If this Deferrable succeeds, ensure that the arguments passed to
 +Deferrable#succeed+ meet certain criteria (specified by passing a
 predicate as a block).  If they do, subsequently defined callbacks will
 fire as normal, receiving the same arguments; if they do not, this
 Deferrable will fail instead, calling its errbacks with a {GuardFailed}
 exception.

 This follows the usual Deferrable semantics of calling +Deferrable#fail+
 inside a callback: any callbacks defined *before* the call to {#guard}
 will still execute as normal, but those defined *after* the call to
 {#guard} will only execute if the predicate returns truthy.

 Multiple successive calls to {#guard} will work as expected: the
 predicates will be evaluated in order, stopping as soon as any of them
 returns falsy, and subsequent callbacks will fire only if all the
 predicates pass.

 If instead of returning a boolean, the predicate raises an exception,
 the Deferrable will fail, but errbacks will receive the exception raised
 instead of {GuardFailed}.  You could use this to indicate the reason for
 failure in a complex guard expression; however the same intent might be
 more clearly expressed by multiple guard expressions with appropriate
 reason messages.

 @param [String] reason optional description of the reason for the guard:
                        specifying this will both serve as code
                        documentation, and be included in the
                        {GuardFailed} exception for error handling
                        purposes.

 @yieldparam *args the arguments passed to callbacks if this Deferrable
                   succeeds.
 @yieldreturn [Boolean] +true+ if subsequent callbacks should fire (with
                        the same arguments); +false+ if instead errbacks
                        should fire.

 @raise [ArgumentError] if called without a predicate