How to use the jinja2.utils.next function in Jinja2

def parse_test(self, node):
        token = next(self.stream)
        if self.stream.current.test('name:not'):
            next(self.stream)
            negated = True
        else:
            negated = False
        name = self.stream.expect('name').value
        while self.stream.current.type == 'dot':
            next(self.stream)
            name += '.' + self.stream.expect('name').value
        dyn_args = dyn_kwargs = None
        kwargs = []
        if self.stream.current.type == 'lparen':
            args, kwargs, dyn_args, dyn_kwargs = self.parse_call(None)
        elif self.stream.current.type in ('name', 'string', 'integer',
                                          'float', 'lparen', 'lbracket',
                                          'lbrace') and not \

# names here.
        if frame.toplevel:
            assignment_frame = frame.copy()
            assignment_frame.toplevel_assignments = set()
        else:
            assignment_frame = frame
        self.visit(node.target, assignment_frame)
        self.write(' = ')
        self.visit(node.node, frame)

        # make sure toplevel assignments are added to the context.
        if frame.toplevel:
            public_names = [x for x in assignment_frame.toplevel_assignments
                            if not x.startswith('_')]
            if len(assignment_frame.toplevel_assignments) == 1:
                name = next(iter(assignment_frame.toplevel_assignments))
                self.writeline('context.vars[%r] = l_%s' % (name, name))
            else:
                self.writeline('context.vars.update({')
                for idx, name in enumerate(assignment_frame.toplevel_assignments):
                    if idx:
                        self.write(', ')
                    self.write('%r: l_%s' % (name, name))
                self.write('})')
            if public_names:
                if len(public_names) == 1:
                    self.writeline('context.exported_vars.add(%r)' %
                                   public_names[0])
                else:
                    self.writeline('context.exported_vars.update((%s))' %
                                   ', '.join(map(repr, public_names)))

def parse_test(self, node):
        token = next(self.stream)
        if self.stream.current.test('name:not'):
            next(self.stream)
            negated = True
        else:
            negated = False
        name = self.stream.expect('name').value
        while self.stream.current.type == 'dot':
            next(self.stream)
            name += '.' + self.stream.expect('name').value
        dyn_args = dyn_kwargs = None
        kwargs = []
        if self.stream.current.type == 'lparen':
            args, kwargs, dyn_args, dyn_kwargs = self.parse_call(None)
        elif self.stream.current.type in ('name', 'string', 'integer',
                                          'float', 'lparen', 'lbracket',
                                          'lbrace') and not \
             self.stream.current.test_any('name:else', 'name:or',
                                          'name:and'):

def parse_print(self):
        node = nodes.Output(lineno=next(self.stream).lineno)
        node.nodes = []
        while self.stream.current.type != 'block_end':
            if node.nodes:
                self.stream.expect('comma')
            node.nodes.append(self.parse_expression())
        return node

self.fail('invalid syntax for function call expression',
                          token.lineno)

        while self.stream.current.type != 'rparen':
            if require_comma:
                self.stream.expect('comma')
                # support for trailing comma
                if self.stream.current.type == 'rparen':
                    break
            if self.stream.current.type == 'mul':
                ensure(dyn_args is None and dyn_kwargs is None)
                next(self.stream)
                dyn_args = self.parse_expression()
            elif self.stream.current.type == 'pow':
                ensure(dyn_kwargs is None)
                next(self.stream)
                dyn_kwargs = self.parse_expression()
            else:
                ensure(dyn_args is None and dyn_kwargs is None)
                if self.stream.current.type == 'name' and \
                    self.stream.look().type == 'assign':
                    key = self.stream.current.value
                    self.stream.skip(2)
                    value = self.parse_expression()
                    kwargs.append(nodes.Keyword(key, value,
                                                lineno=value.lineno))
                else:
                    ensure(not kwargs)
                    args.append(self.parse_expression())

            require_comma = True
        self.stream.expect('rparen')

else:
                node = nodes.Name(token.value, 'load', lineno=token.lineno)
            next(self.stream)
        elif token.type == 'string':
            next(self.stream)
            buf = [token.value]
            lineno = token.lineno
            while self.stream.current.type == 'string':
                buf.append(self.stream.current.value)
                next(self.stream)
            node = nodes.Const(''.join(buf), lineno=lineno)
        elif token.type in ('integer', 'float'):
            next(self.stream)
            node = nodes.Const(token.value, lineno=token.lineno)
        elif token.type == 'lparen':
            next(self.stream)
            node = self.parse_tuple(explicit_parentheses=True)
            self.stream.expect('rparen')
        elif token.type == 'lbracket':
            node = self.parse_list()
        elif token.type == 'lbrace':
            node = self.parse_dict()
        else:
            self.fail("unexpected '%s'" % describe_token(token), token.lineno)
        return node

def parse_mul(self):
        lineno = self.stream.current.lineno
        left = self.parse_div()
        while self.stream.current.type == 'mul':
            next(self.stream)
            right = self.parse_div()
            left = nodes.Mul(left, right, lineno=lineno)
            lineno = self.stream.current.lineno
        return left

def parse_sub(self):
        lineno = self.stream.current.lineno
        left = self.parse_concat()
        while self.stream.current.type == 'sub':
            next(self.stream)
            right = self.parse_concat()
            left = nodes.Sub(left, right, lineno=lineno)
            lineno = self.stream.current.lineno
        return left

def parse_print(self):
        node = nodes.Output(lineno=next(self.stream).lineno)
        node.nodes = []
        while self.stream.current.type != 'block_end':
            if node.nodes:
                self.stream.expect('comma')
            node.nodes.append(self.parse_expression())
        return node

def parse_subscript(self, node):
        token = next(self.stream)
        if token.type == 'dot':
            attr_token = self.stream.current
            next(self.stream)
            if attr_token.type == 'name':
                return nodes.Getattr(node, attr_token.value, 'load',
                                     lineno=token.lineno)
            elif attr_token.type != 'integer':
                self.fail('expected name or number', attr_token.lineno)
            arg = nodes.Const(attr_token.value, lineno=attr_token.lineno)
            return nodes.Getitem(node, arg, 'load', lineno=token.lineno)
        if token.type == 'lbracket':
            args = []
            while self.stream.current.type != 'rbracket':
                if args:
                    self.stream.expect('comma')
                args.append(self.parse_subscribed())