def force_inclusion(trees, otus):
    """Takes a list of TreeNode objects and a list of OTUs as an input. Returns
    the subset of these TreeNode objects where all OTUs within the list are
    present.
    """
    otus = set(otus)
    trees_w_otus = set()
    for tree in trees:
        otus_present = set()
        for leaf in tree.iter_leaves():
            otus_present.add(leaf.otu())
        if otus.issubset(otus_present):
            trees_w_otus.add(tree)
    return trees_w_otus
