code: def get_member(cls, value, default=None):
          
          for member in cls:
              if member.value == value:
                  return member
          return default
docs: Returns the first `Category` member with the matching *value*
          or the specified *default* value if no  member match.
  
          Example:
  
          >>> class Format(Category):
          ...     hour = 'hh'
          ...     minute = 'mm'
          ...     second = 'ss'
          >>> Format.get_member('hh')
          Format.hour = 'hh'
          >>> Format.get_member('day', None)