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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def find_referenced_templates(ast):
 """Finds all the referenced templates from the AST. This will return an iterator over all the hardcoded template extensions, inclusions and imports. If dynamic inheritance or inclusion is used, `None` will be yielded. ['layout.html', None] This function is useful for dependency tracking. For example if you want to rebuild parts of the website after a layout template has changed. """

    for node in ast.find_all((nodes.Extends, nodes.FromImport, nodes.Import,
                              nodes.Include)):
        if not isinstance(node.template, nodes.Const):
            # a tuple with some non consts in there
            if isinstance(node.template, (nodes.Tuple, nodes.List)):
                for template_name in node.template.items:
                    # something const, only yield the strings and ignore
                    # non-string consts that really just make no sense
                    if isinstance(template_name, nodes.Const):
                        if isinstance(template_name.value, string_types):
                            yield template_name.value
                    # something dynamic in there
                    else:
                        yield None
            # something dynamic we don't know about here
            else:
                yield None
            continue
        # constant is a basestring, direct template name
        if isinstance(node.template.value, string_types):
            yield node.template.value
        # a tuple or list (latter *should* not happen) made of consts,
        # yield the consts that are strings.  We could warn here for
        # non string values
        elif isinstance(node, nodes.Include) and \
             isinstance(node.template.value, (tuple, list)):
            for template_name in node.template.value:
                if isinstance(template_name, string_types):
                    yield template_name
        # something else we don't care about, we could warn here
        else:
            yield None