def prune_features():
    """
    Remove features that appear only once.
    """
    global pos, neg
    for k in pos.keys():
        if pos[k] <= 1 and neg[k] <= 1:
            del pos[k]

    for k in neg.keys():
        if neg[k] <= 1 and pos[k] <= 1:
            del neg[k]

    Remove features that appear only once.
    
info.py