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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_checker(func: CallableT) -> Optional[CallableT]: """Iterate through the decorator stack till we find the contract checker."""

    contract_checker = None  # type: Optional[CallableT]
    for a_wrapper in _walk_decorator_stack(func):
        if hasattr(a_wrapper, "__preconditions__") or hasattr(a_wrapper, "__postconditions__"):
            contract_checker = a_wrapper

    return contract_checker