<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def grammar(grammar_type=None, grammar_name=None):
 """Decorator to define properties that map to the ``grammar`` dict. This dict is the canonical representation of the Vega grammar within Vincent. This decorator is intended for classes that map to some pre-defined JSON structure, such as axes, data, marks, scales, etc. It is assumed that this decorates functions with an instance of ``self.grammar``. Parameters grammar_type : type or tuple of types, default None If the argument to the decorated function is not of the given types, then a ValueError is raised. No type checking is done if the type is None (default). grammar_name : string, default None An optional name to map to the internal ``grammar`` dict. If None (default), then the key for the dict is the name of the function being decorated. If not None, then it will be the name specified here. This is useful if the expected JSON field name is a Python keyword or has an un-Pythonic name. This should decorate a "validator" function that should return no value but raise an exception if the provided value is not valid Vega grammar. If the validator throws no exception, then the value is assigned to the ``grammar`` dict. The validator function should take only one argument - the value to be validated - so that no ``self`` argument is included; the validator should not modify the class. If no arguments are given, then no type-checking is done the property will be mapped to a field with the name of the decorated function. The doc string for the property is taken from the validator functions's doc string. """

    def grammar_creator(validator, name):
        def setter(self, value):
            if isinstance(grammar_type, (type, tuple)):
                _assert_is_type(validator.__name__, value, grammar_type)
            validator(value)
            self.grammar[name] = value

        def getter(self):
            return self.grammar.get(name, None)

        def deleter(self):
            if name in self.grammar:
                del self.grammar[name]

        return property(getter, setter, deleter, validator.__doc__)

    if isinstance(grammar_type, (type, tuple)):
        # If grammar_type is a type, return another decorator.
        def grammar_dec(validator):
            # Make sure to use the grammar name if it's there.
            if grammar_name:
                return grammar_creator(validator, grammar_name)
            else:
                return grammar_creator(validator, validator.__name__)
        return grammar_dec
    elif isinstance(grammar_name, str_types):
        # If grammar_name is a string, use that name and return another
        # decorator.
        def grammar_dec(validator):
            return grammar_creator(validator, grammar_name)
        return grammar_dec
    else:
        # Otherwise we assume that grammar_type is actually the function being
        # decorated.
        return grammar_creator(grammar_type, grammar_type.__name__)