def xmlSchema(filename, schema=u'', policy=u''):
    """Given a filename relative to the current module, return an
        interface representing the schema contained in that file. If there
        are multiple <schema /> blocks, return the unnamed one, unless
        a name is supplied, in which case the 'name' attribute of the schema
        will be matched to the schema name.

        The policy argument can be used to pick a different parsing policy.
        Policies must be registered as named utilities providing
        ISchemaPolicy.

        Raises a KeyError if the schema cannot be found.
        Raises an IOError if the file cannot be opened.
        """
