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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 _format_invariants(invariants: List[icontract._Contract]) -> List[str]: """Format invariants as reST."""

    if not invariants:
        return []

    result = [":establishes:"]  # type: List[str]
    for invariant in invariants:
        result.append("    * {}".format(_format_contract(contract=invariant)))

    return result