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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 get_class(classname, all=False):
 """Retrieve a class from the registry. :raises: marshmallow.exceptions.RegistryError if the class cannot be found or if there are multiple entries for the given class name. """

    try:
        classes = _registry[classname]
    except KeyError:
        raise RegistryError('Class with name {!r} was not found. You may need '
            'to import the class.'.format(classname))
    if len(classes) > 1:
        if all:
            return _registry[classname]
        raise RegistryError('Multiple classes with name {!r} '
            'were found. Please use the full, '
            'module-qualified path.'.format(classname))
    else:
        return _registry[classname][0]