from typing import List


def datum_is_sum(datum: int, preamble: List[int]) -> bool:
    """Iterate the preamble numbers and determine whether datum is a sum.

    Args:
        datum (int): a number that should be a sum of any two numbers
            in preamble
        preamble (List[int]): a list of preceeding n numbers where
            n is preamble_size in check_data_for_invalid()

    Returns:
        bool: True if the datum is a sum; False otherwise

    """
    for pre in preamble:
        diff = datum - pre
        # The difference must not be the same as the addend that produced it
        if diff == pre:
            continue
        elif diff in preamble:
            return True

    return False