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PAICE's evaluation statistics for stemming algorithms
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Given a list of words with their real lemmas and stems according to stemming algorithm under evaluation,
counts Understemming Index (UI), Overstemming Index (OI), Stemming Weight (SW) and Error-rate relative to truncation (ERRT).

   >>> from nltk.metrics import Paice


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Understemming and Overstemming values
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    >>> lemmas = {'kneel': ['kneel', 'knelt'],
    ...           'range': ['range', 'ranged'],
    ...           'ring': ['ring', 'rang', 'rung']}
    >>> stems = {'kneel': ['kneel'],
    ...          'knelt': ['knelt'],
    ...          'rang': ['rang', 'range', 'ranged'],
    ...          'ring': ['ring'],
    ...          'rung': ['rung']}
    >>> p = Paice(lemmas, stems)
    >>> p.gumt, p.gdmt, p.gwmt, p.gdnt
    (4.0, 5.0, 2.0, 16.0)

    >>> p.ui, p.oi, p.sw
    (0.8..., 0.125..., 0.15625...)

    >>> p.errt
    1.0

    >>> [('{0:.3f}'.format(a), '{0:.3f}'.format(b)) for a, b in p.coords]
    [('0.000', '1.000'), ('0.000', '0.375'), ('0.600', '0.125'), ('0.800', '0.125')]