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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 handle_activity_legacy(_: str, __: int, tokens: ParseResults) -> ParseResults: """Handle BEL 1.0 activities."""

    legacy_cls = language.activity_labels[tokens[MODIFIER]]
    tokens[MODIFIER] = ACTIVITY
    tokens[EFFECT] = {
        NAME: legacy_cls,
        NAMESPACE: BEL_DEFAULT_NAMESPACE
    }
    log.log(5, 'upgraded legacy activity to %s', legacy_cls)
    return tokens