code: def register_scheme(scheme, constructor):
          
          if not re.search(r'^[a-z]+$', scheme):
              raise ValueError('{0!s} is not a valid scheme'.format(scheme))
  
          if scheme in SimpleConditionFactory._constructors:
              raise ValueError('Scheme {0!s} is registered already'.format(scheme))
  
          SimpleConditionFactory._constructors[scheme] = constructor
docs: Registers a scheme.
  
          :param str scheme: The scheme.
          :param callable constructor: The SimpleCondition constructor.