def METHOD_NAME(stack):
    """
    If only selection1 or selection2 is defined, hbond should run
    against all other atoms as the other selection.
    """
    selection = (stack.res_id == 2) & (stack.atom_name == "O")  # 2LEU BB Ox
    triplets, mask = struc.hbond(stack, selection1=selection)
    assert len(triplets) == 2
    triplets, mask = struc.hbond(stack, selection2=selection)
    assert len(triplets) == 2