def classify(true_otu, pred_otu):
    """
    Classify a prediction as a true positive (tp), true negative (tn),
        false positive (fp), or false negataive (fn).
    """
    if true_otu and pred_otu:
        result = "tp"
    elif true_otu and not pred_otu:
        result = "fn"
    elif not true_otu and pred_otu:
        result = "fp"
    elif not true_otu and not pred_otu:
        result = "tn"
    else:
        raise ValueError("this should never ever happen")
    return result