def validate_mutex(mutex)
      return if mutex.respond_to?(:lock) && mutex.respond_to?(:unlock)
      return if mutex.respond_to?(:unlock!)
      raise TypeError, "'mutex' must respond to ('lock' and 'unlock') or 'unlock!'"
    end
def validate_mutex(mutex)
      return if mutex.respond_to?(:lock) && mutex.respond_to?(:unlock)
      return if mutex.respond_to?(:unlock!)
      raise TypeError, "'mutex' must respond to ('lock' and 'unlock') or 'unlock!'"
    end
Validates that an object behaves like a +::Mutex+
 Must be able to lock and unlock +mutex+.
 @api private
 @param mutex [Mutex] mutex to be validated
 @raise [TypeError] if +mutex+ does not behave like a +::Mutex+
 @return [void]