import collections


def merge_dictionaries_with_list_values(dictionaries):
    """
    Merge dictionaries with list values, concatenating the lists.

    Returns a dictionary with keys in the union of the input keys.
    The value for a particular key is the concatenation of the values for that key
    across all input dictionaries.
    """
    result = collections.defaultdict(list)
    for d in dictionaries:
        for k in d:
            result[k].extend(d[k])

    return result